Okay, just a few things I found with a couple hours of play time with a friend.
Firstly, it's been said before, ladders sometimes propelling you into Narnia, that kinda sucks
Second, i (host) cannot hear p2s gun shots
Third, occasionally while loading an area, all backpack items vanish
Fourth, I was climbing a ladder, touched part of the roof with my head and combusted (overall ladders feel weird, in my opinion the old ones were much nicer)- obviously this is not co-op exclusive.
Fifth, tides can de-sync leading to one player getting killed and the other shortly surrounded by respawned enemies, this one is likely one of the most troublesome bugs.
Sixth, it's a classic, we all love it (likely not just for co-op) weapons and items falling through the map.
Seventh, items transfered between players will become invisible for one or both players, this includes mission items and artefacts.
That is all for now, overall I've been loving the game thus far, great job to everyone over at cm games, because you're doing amazingly.