I agree on the disparity in abilities being a devaluer of them as a concept. I like the mystery and some of the implications, but some of the writing of their returns has taken the 'wow factor' of it too far into ehhhhhh moments.
I like that it's implied that Ollanius can live forever unless he's killed; he seems in genuine fear for his life at points. I like that he's 'old-age' proof but can simply be killed by a bullet to the head or whatever.
I also like that Grammaticus seems to need the Cabal and some time and effort to activate his resurrection ability. He seems to stay in the same body and return to life when his magic button is pressed. Only now he's burnt out the circuit and pressing that button won't do anything anymore.
Those two abilities seem clear - immortality unless killed, resurrection that needs a specific set of circumstances. Strengths and weaknesses.
Where it loses it for me is Matt Damon and the Guardian woman.
Matt Damon gets melted to ash and slurry, then reappears hardly anytime later completely whole. Did he regenerate from a pile of gloop? Really?! Come on. So then what? The Cabal can magic him into existence? In no time at all anywhere they like? pls no. He continues the Culture agent thing and has back-ups that can be sent in after him? Ehhhh.
It's all too magic-y and removes weaknesses if the guy can come back from being 14 molecules short of being atomised twenty minutes later. It's not made clear that he needs the Cabal, unlike John.
Guardian woman gets eviscerated then - correct me if I'm wrong - appears on a ship that left before she died, right? Totally whole. So, does she have an athame/teleport device? Those get around. Or can she, like Matt Damon, somehow choose to pop back into existence/take over someone else, Lucius style? But without the Cabal? HAX.
Vulkan was okay at first - he regens in the same body, super-Wolverine style, and it appears to have debilitating effects. Either that or even his Primarch mind has just broke from repeated torture, travelling through the warp unshielded and burning up in an atmosphere and surviving a meteor-like re-entry.
But he starts coming back from death in minutes. Totally cheap. And it was utter melodrama to kill him like 3 or 4 times in that one extended fight scene just because he could be killed with no ramifications.
However I don't have a problem with the Emperor 'dying' like he did as a) we haven't seen the series version of it yet and b) I think there's plenty wiggle room (as already mentioned above) to get away with it.