I think the worst part about the campaign are the early swarm missions where it's hardly a test of skill and more like a gear check.
The later missions where the hordes start and stop coming from one direction and force you to react were more interesting, but even those suffer from horrible design. For example on the last swarm mission leading up to the final base mission, you have 60k infected coming, of all types except giants. You have to micro super fucking hard to stay alive for many minutes there, I think at the end I was missing thousands of Titan HPs in total. But at around 40 to 35k infected remaining, the special infected are all dead and now it's just an endless stream of regular infected. If you still have enough troops to gun them down, you can go afk for 20 minutes here. It's pointless.
The game would be better if this stuff weren't part of it. That's harsh, but true.
I think hero missions are easier to salvage. Your hero is actually pretty shit, I'd prefer a Titan or something. Even five soldiers would probably do a better job. The fact that regeneration is disabled means Caelus description of face tanking as he shoots infected is bullshit.
But the worst thing imo is their ham fisted band aid: Infected are deaf and inactive as long as you don't see them. They don't wander around and they don't react to noise. I assume they did this because it was the very only way to even make the higher difficulties playable. Otherwise on the open missions you'd just get fucked up after firing your gun 20 times. The downside here is: you get punished for exploring, and Vision increase skills are detrimental as well.