I actually like it when they are all different levels because you get level ups multiple times as often. Helps break up the monotony.

One of my pet peves is bosses that get inordinately more difficult as they get closer to death. Especially if they show a health bar. What is the point in a health bar if you cut the damage in half for the last 1/4 hp?!? Why couldn't you just give the boss 25% more HP and have them do their super attack at 2/5 instead of 1/4. It just seems like lazy game design the make landing the final blow have an artificially inflated sense of accomplishment.