I bought it shortly after playing the demo. I'm not too far in, well actually I'm at the start of level three (out of five total). Even on normal the game can get pretty tough. Like someone said, it does share a lot of similarities with Shadows of the Damned, atleast tonally, and even Dark Souls (the imminent threat of failure is similarly pervasive). It'd give it my recommendation for sure.
As far as Heroes of Might & Magic goes I love Two and Three, but as you mentioned Four is kinda... well not as good. I played these back in the day though so there's some nostaliga involved for sure, but I still think they might hold up if you're willing to give them an honest try.
As for Might & Magic the only one I every played was VI: The Mandate of Heaven, but I remember having a terrific time with that game also. Specifically I remember that game being super tense at times.
Thing is though, you really got to look past the dated graphics with these games. Hell, I find them charming, but someone new to the series might be put off. Hope you have fun with them!
Valley of Defilement is worse, by a lot, for a few reasons. It's longer, you're generally fighting above a bottomless abyss and your right, that big open space in the beginning of the second act is horrible.