This is a hard one. There is no doubt that the reformation had a huge effect on European and World history, not only changing religious practices but the society, education, politics. In that respect it totally deserves its place on the list. The great question is how you represent a spiritual and intellectual movement with a building or a group of buildings if you do not want to downgrade it to an intangible heritage.
How to represent such a complex movement stretching over many countries and several centuries? UNESCO or rather the German nomination chose to concentrate on the probably most famous reformer Martin Luther. The 95 theses that he allegedly hammered at the portal of the castle church in Wittenberg may be the most famous single moment in the history of reformation. So far, so good.
There are several problems with this choice: Luther was certainly not the first protestant: Men like John Wyclif formulated basically all the ideas and church criticisms as Luther but almost 200 years earlier! A bit later Jan Hus had a huge, even political impact. Similarly to Luther who was ordered to Augsburg to renounce his beliefs Hus was ordered to the council in Constance with a safety promise but then burnt on the stake. (The council of Constance was – with and beyond the thrilling story of Hus - such great event of incredible influence that you could very easily make a good WH nomination out of this! And much of the original buildings are preserved). There is certainly an argument that their similar ideas and action at a much earlier time would give them preference over Luther. Then there are Luther’s important contemporaries like Zwingli and Calvin, of which the former was often ahead of Luther’s actions.
If you still decide that Luther is the best choice if you want one single person to represent the reformation the question remains you to represent him materially on the WH list. UNESCO may not have chosen but it accepted two buildings in Eisleben and four in Wittenberg.
I visited all of them and the sites in Eisleben that were nominated for an extension but are now removed. The Town of Eisleben was a mining town and this was the reason why Luther’s parents moved there from the nearby town of Mansleben (now also nominated for the extension). Nowadays Eisleben is a rather small provincial town, widely preserved but not outstanding in any respect except its Luther connection of course. Fun fact: Luther’s parents’ name was “Luder” means today hussy. Probably it had the same meaning at Luther’s time since he felt the need to change his name!
Both his birth house and has death house are included and open as museums. Both museums are worth visiting, offering a lot of historical introduction, family history, Luther memorabilia. You can even visit his living room, his cradle and his death bed. But they are all fake and for several and strange reasons: Shortly after Luther’s death people started to flock to those places and broke wood pieces of his death bed as tooth pics!!! because this was supposed to help against tooth ache! Imagine: Luther (and of the reformers) got rid of the cult of saints and only Jesus should be the mediator to God and now his followers made him into a saint and his remains into relics! The authorities did not approve of this practice and burnt the bed and other earthly remains. Then around 1600 there was a big fire in Eisleben that destroyed most of the Houses, most of them probably half-timber constructions. For a long time the Luther houses were neglected and nearly forgotten and only in the 19th c. when Luther was reinterpreted as a national hero his houses were rebuild. The birth house stand supposedly on the original spot but is almost a complete new building: At the time they had no archeological ambition but wanted give a good idea how it had looked at the time.
Even more difficult is the situation with the death house: Also this is a romantic reconstruction but it is hotly debated if this is even the right house. The latest knowledge seems to be that the real death house stood at the place were today stands the Hotel “Graf von Mansfeld”, where I spent my night in Eisleben. This is also not the original building but was build after the great fire in 1601 and strangely you do not find any information about its connection to Luther anywhere in the building.
There are three churches in Eisleben that have been nominated in 2015 as part of an extension. The first is the church of St. Peter and Paul where Luther was baptized. The church bay be the best visit in Eisleben: It is very freshly renovated and seems largely authentic but has a modern addition: a round whole in the floor with a water pool where people can be baptized with the whole body. This makes a strange but alluring contrast, first architectonically and second because Luther was a strong adversary to adult baptism (like Zwingli). The second church is the town church of St. Andrew: The chancel is supposedly the original of Luther’s last sermon (despite the catastrophic fire of 1601) but I found it the least impressive of the three. The last is St. Anne’s church: Interestingly a monastery that was build on Luther’s initiative! It is quite grand with large ceiling frescoes, a wonderfully boorish chancel, uniquely made from plaster, and a famous “Stone bible” with creative reliefs from the old testament. For an additional 2 Euros you can also see the former monastery cells from Luther’s time. For such a small time the group of three rather large churches is impressive though I do not thing they would add any OUV to this already rather unconvincing group of buildings.
So one building is a reconstruction on the original spot, the other one a reconstruction on the wrong spot. There is hardly any authenticity. But I thing the problem goes further: If this site should represent Luther as the most important reformer should those houses be chosen? The personal cult they represent is not only alien to the protestant church but also to UNESCO.  Imagine they would inscribe birth, living and death houses of important people. There would be no end to this. And there are certainly birth houses just as popular as Luther’s: Think, they would inscribe the houses of Mozart, Shakespeare, Einstein etc.  Much more important seem the places that were important in his life as a reformer. Of these the castle church in Wittenberg seems the only one that is inscribed.
Luther spent extremely little time in Eisleben: His parents moved shortly after his birth to Mansfeld but Luther loved the region of his youth dearly while he seems to have disliked Wittenberg, even calling it dirty. Today Wittenberg makes a much grander and livelier impression then Eisleben. There are lots of restaurants and several museums. The most important spot for the reformation is certainly the castle church and its famous portal.  The building is still the most impressive site of this rather pretty town but there is hardly anything original about is: The church was destroyed several times and is now a neogothic building or even the reconstruction of a neogothic building. The oldest parts seem to be some tomb stones on the outside of the church wall. The famous portal is totally new creation. Of the further three buildings the town church seems to be better preserved and contains some interesting tomb stone and altars from the period among them the famous Luther altar with Luther sitting at the last supper as one of the disciples (though I could not find out as which disciple). The Luther house where he lived with his wife for decades and taught was closed for renovation but there was an exhibition in the neighboring Augusteum telling the story of Luther’s life with some supposedly original memorabilia like his beer mug. The Luther house is also disputable as WHS, but it has at least the advantage of being more authentic then the houses in Eisleben. The last included building is the house of Luther’s important colleague Melanchthon. The building at large seems original while hardly any of the original interior is preserved.
But Wittenberg has more to see then the four sites: There is a nice old monastery museum about the history and importance of Wittenberg before Luther. It has very few original exhibits but recreates the monastery church impressively with light and projections and is very informative (if not a bit overwhelming). It puts Luther’s time here in a wider context. Very interestingly Luther’s great protector (there would be no Lutheranism today without him) Archduke Frederic of Saxony hat one of the greatest collection of relics and Wittenberg was therefore a great pilgrimage site!
Then there is a big modern Panorama. Quite nicely made it retells the story of Luther and the reformation. As an introduction this is good and impressive (if rather expensive), if you know your history you can skip it. Very interesting is the Cranach Museum. The paintings and woodcuts of Lucas Cranach and his sons had a great influence on the spread of the Luther’s popularity and influence, comparable to the influence of Gutenberg and book printing. He was for Luther and the reformation what Holbein was for Henry VIII: He gave them a face that make them popular until today. Cranach was so successful that he was mayor of Wittenberg for many years and owned about a dozen houses all over Wittenberg. Only Dürer was comparably successful (though certainly the greater artist). If you wander through the streets you find plaques about people who lived and studied here and the university: among them the famous Johann Faust, the great German poet Lessing and Count Nikolaus Ludwig von Zinzendorf, who founded the Herrnhuter Community.
While the single buildings also here in Wittenberg certainly lack OUV in an architectural sense the whole town is a quite fascinating town capsule, good to spend a day if your interest goes beyond ticking off. To tick it off three hours are enough. Perhaps an approach to inscribe the old town of Wittenberg as an center of knowledge and a cradle of modern thinking would have been more convincing then the single buildings. Another approach could have been to concentrate on the places in Luther’s live that were turning points for him and the reformation. This seems more convincing to me but also offers great problems of authenticity:  There would be first the church door in Wittenberg with the described problems. Then certainly the great Diet of Worms: This took place in the bishop’s palace next to Worms cathedral, but nothing remains of the palace. The most important place may be Augsburg where Luther met Charles V, read the famous Augsburg Confession and where the Peace of Augsburg was settled. These were key moments in European history and you wander why Augsburg with his many attempts to gain WHS status never tried this path! The reason may be that Fugger palace were the Diet took place was destroyed in WWII and only has a modern reconstruction. Finally, Wartburg castle, where Luther translated the Bible from the original texts but this is already inscribed.
None of the approaches are totally convincing. Of the inscribed towns Eisleben is only for the die-hard and the history buffs while Wittenberg is really worth is if you are not blinkered by the limits of the inscribed sites.