It's true that it's not as scary as the previous Penumbra games, but if you don't excpect
anything from it, you will get more fun out of it that way.
For example, I wanted more puzzles after I completed Black Plague... and there it was: Frictional Games announced Requiem.
Trust me, I read all the stuff people wrote about the game being not scary at all and that it's the most worst game in the whole series.
Thanks to that, I wasn't waiting any horrors from it and I was playing
only for the puzzles.
That's why I got almost heart attack while playing it.
But as answer for your question:
It has some spoilers regarding the end of both, Overture and Black Plague.
But other than that, it's only puzzle game.