Never mention a song of ice and fire!! Martins writing style is pure shit, and his story is going no where. I hope he dies before he finishes the series.
Cain survived those encounters he didn't kick their asses.
Space marines are the best of the best, but now a single space marine isn't good enough compared to some figures. Even with being genetically superior with the best weapons against something like a big tyranid, they are screwed.
As for Martin's work- your opinion's your own, but the outstanding sales and popularity seem to argue with you. Personally, I think his work is great.
Cain- surviving those encounters is pretty damn kickass. Most people wouldn't be able to run far enough to piss their pants before they were dead, and he fought on and killed the enemy. For enemies that powerful, surviving with them dead is kicking their ass.
I think the whole thread has a point, if you make all the enemies too ridiculous it can be a problem. But in a world whose own writers say is just a story of how the Imperium slowly dies and fails, it makes sense to have enemies that can crush humanity, and do. It also makes sense for space marines and other such servants of the Imperium to do their duty and fight these threats. And finally, it absolutely makes sense that they succeed sometimes, despite the odds.
Not saying there isn't any fault within the entire spectrum of 40k fluff, but the large majority I've seen isn't a walking Mary Sue collection.