Though Hubert's review is over a decade old, it was the most recent one written before my May 2025 trip to Vall de Boi and I have to say, not much has changed. In fact, Hubert's review is quite comprehensive and I recommend giving it a read before reading my own here. I'll try and fill in any gaps.
Yes, this high concentration of churches were constructed by the Lords of Erill, though as the attendant at the one of the churches told us, it was not for religious piety that the lords constructed the buildings but rather to establish a particular small territory as their own during the constant conquerings and re-conquerings between the local leaders during that period.
In our visit to Vall de Boi, because it is so remote and takes so much effort to get to (we rented a car from Barcelona) there were very few other tourists, leading to the sense of peace and serenity that other reviewers have described. It's practically just you and the majestic mountains, deep in the heart of the Pyrenees. (Though Els' review makes me wonder if it used to be more popular.) We only saw one proper "tour group" in our time in the Valley, and they all arrived on a bus at the same time as we did to flood the seats of the "audiovisual" projection show in Sant Climent de Taüll. (Which, while not amazing, is still pretty neat and worth seeing.)
Eagle-eye readers may notice there is a small discrepancy in some of the reviews, where Hubert mentions that outside of the four churches in Boí, Taüll, and Erill la Vall, "the others are not regularly open to individual visitors" but other reviews from the same year (Els & Clyde) mention visiting two additional churches in Durro and Barruera. (Which also list as being open year-round on the website.) The only explanation of this that I can give is that both are right... when we arrived during the "open season" for these two additional churches, we were told in-person that they were closed that day by the attendant selling us tickets. No other explanation given.
Beware that finding something to eat in the Valley can be very challenging! We arrived in Boi at 4:00pm, a very typical Spanish lunch time, only to find that the kitchen in all of the restaurant/bars were closed, despite each business being listed as "open" on Google Maps. We tried four different locations but everyone told us they were closed and had no suggestions for where we could eat in town–even the tiny "general store" was closed for a siesta! Everything was to re-open at 8:00pm-8:30pm so take this into account when planning your trip there.