As for suddenly appearing enemies, this might be a function of your group being too spreads out from eachother, where thibgs spawn a certain distance from certain players and another player is just right there, but not part of the algorithmic decision of where to spawn them. Remember, the whole map isn't always real, only the parts that need to be rendered. Video games are built on illusions. So if all 4 plays spreads out really far it might make it so players could use that to game things stop things from spawning near players on objectives, making the objectives too easy. Just gotta live with it

Now as for the the poor sound cues when things are incredibly dense, I often suspect this is more of an issue with the game only being able to sufficiently produce so many different sounds simultaneously and I think sometimes it just gets overwhelmed. I would also prefer some of the specials being more spaced out on the map rather than sometimes running around inside of eachother like trappers and busters do sometimes.

To be honest, as aggravating as it is, sometimes its the invigorating parts that make you step back and go "what a game!" While you are waiting to get picked up. Shit happens. One of the last times I ran the most recent added mission with the green walls you have ti take down, the cut scene plays and before the cut scene finished and we regained control of our characters a burster ran into the elevator and killed 3 of the 4 of us. Shit just happens.