Risk vs. Reward.
If that back room is spawning 20 force-field allies with items that regen making you immortal, chances are that isn't what was intended.
If that back room is spawning a HUGE ambush where you very much can get killed which is why the author put it there so you would flee from a huge angry mob, well, that's just awesome!
If you just so happen to have built a character that can stand in the middle of that enormous mob and take a beating without the assistance of everything, perhaps it isn't AE that is having the balance problem.
The room is pretty much death. There are so many mobs in there you can run across the room on top of the heads of the mobs. It's kind of fun until you get killed. Using inspirations to cap my defense and resists, I only last about 30 seconds.
From your description, it sounds like it is not working as intended. If you can get that map to perform the same with your own settings, you might want to report it as a bug in MA.
I wouldn't report your buddy's map, just in case the communication wires get crossed (wouldn't want someone to accidentally flag his arc as "bad"). However, filing a bug report explaining exactly what settings/map cause the problem could be very useful to the devs.
I've been working with AE for a while now and have made some very good missions, so I have some experience under my belt. I ran a buddies mission the other day and I'm trying to figure out how it works.
Map Type Large Longbow Map. 19 ambush / boss spawns 22 ally / defendable objects (4 front /13/5 back)
According to the map details you can split up where things spawn with a cap of only 5 defendable objects in the back. How ever ... If you setup everything as "back" the entire back room has all 1000 enemies, allies and objects in it.
Is this a bug or is this working as designed? The map states you can only have 5 allies in the back, but if you make 22 allies and say "back" for all of them, they all spawn in one room. Am I supposed to break up the allies into different categories, one for back, one for middle and one for front and specifically configure the size of each one to match the map cap?
I would expect the map to just spawn 5 allies in the back and that's it. But all 22 allies with their defending groups all piled up in one room show up which is crazy.