I don't really know how to explain this HUGE issue...
So I'll make an example:
you start a match (public or private) and everything seems ok, your mate lags a bit but let's say that's normal.
After the first round you see him get hit a lot and he goes down, BUT HE IS NOT!
The other player is like playing another match, if he jumps i see him jumping and the same if he shoots but he is in a different place in the map and the same happends from his point of view while he watches me.
So it starts being out of sync, like if we were playing different matches...and when i defeat an enemy wave he sees he's still fighting it.
So at a certain point you see your teammate shooting at walls and going down and you can't help him... and if you don't you both get disconnected.
It's annoying (and hard to explain, since i have never seen something like this in a coop game)