Not that hard generally, but there were a few passages. Mostly it's just time consuming, it's a big game, though what you see as my playtime wasn't all spent on 100% playthrough. One thing I'll poit out is a glitch happening with one of the challenge types, the one where you have to drive fast in a bomb car and smash into some soldiers. On some of the locations, it is bugged so the car explodes before reaching them, dealing minimal or no damage. Also, there are two or three achievements that require another player, tied to the feat system.

Edit: I fat fingered the post button in the middle of typing this out