I am level 21 right now and until level 19 it was utterly impossible to manage tier 3 missions. Especially because a lot of lower level players (I think level 5 or so even) try them out. I just managed to survive my first tier 3 mission because I had well equipped, even higher level characters (boltguns, flamers, chain axe, power sword, some sort auf autocanon) - the weapons should have a rating of about 300 to start there.
There should either be an indication if a tier is too difficult unless the rest of the team is really good (maybe the matchmaking algorithm can take care of this) or the mission should be locked. Going from tier 1 to 2 was ok I think (dunno, but I guess around level 10 it became doable), but 3 had such a gap due to higher XP required for leveling that the game felt repetetive (especially with only 4 unlocked missions in the moment).
Then: if the whole team is out of action, like pinned down, captured, dead, whatever, the game could end immediately. At the moment we all have to watch how the pinned down characters slowly lose their life but there is no chance for them to come free. If it were possible to break out of capture or getting pinned by a pox dog or so, that would be interesting.
Sometimes the game mechanics feel kind of unfair: three people down, the last one is about to revive a team mate and then like out of nowhere a special enemy spawns and smashes the guy.
And I had this situation a couple of times where an armored Ogryn enemy attacked me and because I had no chance of damaging him, I tried to escape but he finished me, because he seemed to be faster and had long reach and I guess after one dodge he just oneshot me.
The staggering mechanics are a bit rough. Deciding to turn and run is always fatal because you start sprinting, get hit in the back, immediately decrease speed and then get finished off.
Also: for whatever reason enemies tend to spawn behind you. Sometimes just one dude. Maybe we all don’t recognize them while waves are thrown at us, but those single tiny dudes really suck with the staggering mechanic in place.