So, I clocked it last night on 'medium'... And I'm probably half an hour away from clocking it this afternoon on 'hard'. Funnily enough, 'hard' isn't even that bad; most sections have actually felt easier. I think it's because this time around, I'm playing far more liberally with my ammunition, i.e. no more Lascannon ammo is usually indicative of everyone being dead.
It has been 10x more fun, as well. 'Hard' is defnitely the go. My main gripes still exist, though: too few executions and a severe lack of variety in the environment. Regarding the latter, it's as if they've purposefully chosen to not show off the wonderful engine, crushing all the colours and detail. It's amazing how much better a space marine looks under a blue sky.
Other than that, the shooting and combat feels excellent. The Stalker Bolter is immensely satisfying to use and there's nothing like slotting your chainsword into a CSM's head. It'd just be nicer if there was even one more execution animation on the CSMs/daemons. I mean, there's 2 for each weapon on the Nobs/most enemies, but there's only one for the CSMs/daemons, if I recall correctly.
All in all, a very solid game that shows just how far solidarity/consistency in a game's universe and presentation go. Take heed, other developers!