My impression is that *most* of these are not worth the opportunity cost for most builds. A lot of builds are relatively feat thirsty and can't just drop a feat in, and while *most* of the tradeoffs are worth the tradeoff directly, they aren't worth missing out on a traditional feat pick. It's also very hard to evaluate the tradeoff, even for veteran players, if we just know an effect has "a small chance"- a small chance could be 1%, it could be 10%, those are very different in terms of economy to the point where a feat like Echoing Soul might be "worth it" on a 1% chance for the saving throw bonuses but "not worth it" on a 5% chance depending on how the effects are distributed.
However, just being "worth the tradeoff" isn't worth it when you look at the choice between these dark gifts and taking just a plain old normal feat. The opportunity cost of taking a feat that doesn't prereq your high level feats and comes with a drawback for benefits that are, to be frank, mostly fine but replaceable with gear, entirely optional for your build, or very niche just doesn't quite move the needle on builds. I can think of a few builds where these feats might find a spot, like Mist Walker or Touch of Death.
I feel like the economy around these feats neglects how severe the cost of missing a feat is for most builds. If this was an *optional* feature for characters to pick in addition to other feats, the actual feats themselves suddenly become a lot more palatable, but otherwise I think some of these feats need some tuning as well as more transparent information on proc rates. I will never pick a build option that tells me there's a "small chance" of something that could get me killed happening without knowing the proc rate. Immobilize at the wrong time can be absolutely devastating, and while I would love to have a huge boost to saving throws being afraid/blinded/immobilized in a room full of mobs can end a run. Especially if there's no cooldown and they can chain, since being immobilized gets rid of mobility options.
Anyway, I have a very hard time, as these feats are, possibly justifying axing a feat in any of the builds I'm familiar with to take any of these, and while I could see making a new build around a couple I'm not confident they'd be, well, any good.