I find cover is absolutely the worst way to play. I die way more often if I try to pop in and out of cover, and for the most part it's ineffective. Maybe they're doing something different than me. Are they using lock-on aiming (full or soft lock)? I'm playing with free aim. When you're in cover, you don't have an aiming reticle until you press LT to aim, so you can't line up your shots before popping out. You also can't jump from cover to cover with a button press like in most cover shooters. The enemies are accurate shots, so standing still for a second can get you killed pretty quickly. It could be that you can abuse cover in combination with bullet time, but I find standing still, even in bullet time, gets you killed quickly. For me, the best way to play is to be very aggressive and keep moving so the enemies can't draw a bead on you. Headshots recharge your bullet time quickly, so once you're in bullet time you can keep it going if you're accurate enough.
I'm pretty far into the game now. Chapter X or XI, I think. The basics of the combat do not change, and the aren't really many differences between the enemies, other than weaponry, armour and grenades. Some enemies do seem to be more accurate at shooting than others, and are more effective at taking cover. The untrained gang types pretty much stand in the open shooting wildly. The game is surprisingly long compared to the previous games, so I imagine if you aren't really into shooters or have found a way to abuse cover, it might feel like a grind. For me, the combat is so visceral and stylish, that I never really get bored of it. Any time I'm in an office area set-piece with a lot of glass, I get pretty excited. Mowing down endless goons is always amusing. Last night I broke the 1000 kill mark, and I'm not even finished the campaign yet.
I would describe the game as a real hardcore shooter. There are a few sections that I found fairly challenging, where I died quite a few times and had to retry. That's on normal with free aim. Hard would probably be very difficult. Comparably to other shooter type games, the normal setting is much more of a challenge. Most shooters on normal are incredibly easy.