That's only a select few of VN purists....Most are pretty much in the same boat as you
VN players agree that Ufo adaptation is better than DEEN's but they have a LOT to ciriticize the anime since they have something to compare it to as oppose to Anime-watchers only. Typemoon's fans are a perfectionist bunch. They'll complain if they find even a miniscule mistake. This does not exclude me as well.
I fought against the hater purists for weeks on MAL....There's this guy called Fai (CookingPriest on MAL) that is the worst one that I've ever seen. Like seriously this guy has consecutively hated on the second cour week after week and to make matters worse he even went as far as forcing his opinion on anime only's and speaking for us as if we all agreed with him or were too stupid to actually pay attention to and analyze what's going on in the episodes