It's not Ravenloft, so much as the horror genre. Some people find drow to be inappropriate protagonists in a horror adventure. There was a section of one of the 2E Ravenloft books (or, maybe Dragon; it's been a while) about genre appropriateness of non-standard races. It has nothing to do with power level, whether it's "logical" for them to be taken, or anything of the sort. It's a meta-game thing. Unless, of course, Ravenloft chooses based on tropes, which isn't out of the question.
That's a very personal, taste-driven thing, though. I wouldn't fault anyone for allowing oddball PCs -- as long as the group is having fun. It was just a call-out for consideration related to the original question.
Personally, I'm not sure where I'd draw the line. Somewhere before myconids, for sure. I don't allow drow, anyway. Kender are obnoxious and very mood-breaking for horror; if a player was strongly interested in playing one, I'd have to ask myself whether they were actually into the "mood" side of a vampire lord or if I should just run the adventure as a dungeon crawl. 3E-style tieflings would be fine. 4E/5E tieflings would be a harder sell, as would dragonborn, but I'd give them a shot. Tinker gnomes would make kender welcome, by comparison. That's roughly my line -- I tend to prefer human-centric settings and parties at all times, though. Someone who likes Talislanta is going to feel very differently, and that's cool.