I think it would have been a +1 if they had PvP. Deathmatch or some type of capture the flag. I know the squads are small, but they could do it just like Gears.
It would be impossible to balance with all the powers like singularity, stasis, throw, cloak, biotic charge, cryo blast, combat drones, and KROGANS. I don't see any way PvP could be done without being a chaotic mess or every class being overpowered. Unless they only had the soldier class for PvP, but that would be Mass Effect at all.
The combat is pretty much perfect for the horde-style mode. I think A-to-B objective maps would add some variety though.
I think it would have been a +1 if they had PvP. Deathmatch or some type of capture the flag. I know the squads are small, but they could do it just like Gears.