It’s a tricky line to walk, right? On the one hand, options are always good, and options for significantly de-powered PCs would be highly appropriate for a horror toolbox book. On the other hand, how far can you take it before it feels like “why are we playing D&D instead of CoC in a fantasy setting?” I think the Survivor rules strike a nice balance. A great option for one-offs, the idea to use them to play out flashbacks and nightmares is awesome, and if you want super low-power PC options, they fit the bill.
I think one of the strengths of D&D and especially 5e is that it can do multiple genres and flavors decently without going bland. Most horror or dark fantasy RPGs deep dive into the genre and only work one way. With Ravenloft, they could have explored Ravenloft as Dark Heroic Fantasy, Dark Swords and Sorcery, Dark Grim Fantasy, Dark War Fantasy, and Dark Mythic Fantasy. By not offering the dial, they kind of fell into that trap they were trying to avoid.