I liked Caiphus Cain, but he suffers the same fate as Gaunt's Ghosts. They are only imperial guard so the writing was specifically designed so that the characters will only come up against and fight characters that are realistically in their reach of killing.
Caiphus and Gaunts ghosts will never take down some greater demon or a necron lord etc.
Just to add a note here, a bit off topic.
Is this a growing trend here for scifi/fantasy?
You have popular tv shows/movies where the only way a villian can be defeated is if the hero becomes just as powerful. We have the typical super hero shows where the good guys have to be able to match the enemy in strength as usual or by teaming up and by combining become just as powerful, then you have very popular anime series like DBZ where the character trains and ups his strength and is now able to blow up a planet.
I like to call this the Hulk effect, essentially a character that cannot be killed. The more you hit him, the more bad ass he becomes.
I don't think you can blame Workshop for having fluff about super human characters who have the best stuff to kill the biggest bad ass enemies, it's what people want. If you want stories about the little man doing things read Bernard Cornwell or something similar. The whole point of the grim dark is that you can't do anything as one person. Sure there are exceptions to the rule like Gaunt and Cain, and to be honest, they make the best books. This isn't a new trend for black library, they've always done that, space marines by their very nature are the best of the best, sir, with honours!
All I'm saying is you have to have realistic expectations on the type of genre that you're reading.