1st problem mobs from different floors can cast spells on you. Imagine getting nuked and you have to run all the way around just to smack their face.
2nd problem Due to the fixed camera angel, you cannot see how many mobs are in the area.
3rd issue, see video, but mob AI gets stuck very easily on corners, including stairs. I feel this was key to how we managed to kill the boss.
I like the claustrophobic design, but it's just not practical.