I love the Ravenloft setting, but I have to admit that it is really hard to pull off horror games with Dungeons & Dragons. The biggest issue is clerics. They can heal everyone, possibly resurrect the dead, and curb-stomp the undead. Even if clerics are heavily limited, mid to high level fighters and mages can handle the vast majority of monsters.
I can see why level-draining monsters were added to fight this, but I think that mechanic is very unfair (not to mention a terrible paperwork mess). In any case, I've done what I can to make it scary for the players, but oftentimes, it is not an easy sell.