I would be more excited to see them in the game as an NPC for PvE. As for mechanics they definitely need a resurrection mechanic. Damage them enough and they will 'power down' and become inert, then you must do enough damage to it in order for it to 'phase out' or it will stand back up with maybe half hp. Of course then wraiths have the ability to completely 'dodge' some of your attacks

There best be a dungeon or instance with them starring. The thought of being ambushed by flayed ones/wraiths or shooting and stomping through swarms of beetles while fighting to get inside a Necron tomb to encounter the Necron lord... Well, that would just be one of the most awesome...

Having read this thread again and thinking about it some more.

I think a much higher hit point base or perhaps just increased toughness sounds right. The offset could be that the more they are beat up the less effective their weapons are...i.e. systems shutting down until they become inert so to speak as another poster mentioned along with a limited health self rez. if they are downed again before the rez cooldown is up then the cooldown doubles for example.