The interesting thing about Mega Man bosses for me personally is that rarely do I find them just right, the porridge is either too hot or too cold more often than not, is what I'd say if Porridge man was a thing...wait, I'm losing my point here.
Damage output wise most seem to be far more deadly bumping into you, I imagine that was so you didn't just stand next to them firing rapidly and abusing I frames, as a result it's kinda odd that some of them aren't much of a threat weapon wise but god help you if they're erratic springy bastards.
Mega Man can barely jump over Robot masters without perfect timing to my knowledge if they're not moving towards you which can lead to some cornered situations where you eat a lot of damage.
Yet you also get some RM who go down with ease before their weakness is even factored in, obviously the non linear nature of selecting stages factors in here as well.
MM2 in particular is an odd one, putting aside out easy "normal" mode for a second, when on hard you still get that odd mix of breezy and blitzed. Metal Man is simple but interesting in how he's reactionary, his attacks are based off of your own if I recall, his weapon carves through many other bosses (among half the enemies to boot). Flash Man is another easy one but he's a big ol' gimmick based around trying to speed blitz him.
Meanwhile there's that ol' meme song about "I can't beat Air Man" for a reason, him and Wood Man hit hard with a projectile barrage that's like another level compared to the previous jobbers.
As the series goes on the tropes repeat themselves, some of the bosses are a simple bit of pattern recognition, some are manipulation based, others are mad sprints to the kill but even among these tropes the challenge and damage they dish out and take can be all over the place. Ultimately I never really look forward to MM bosses, the obligatory boss rush endgame bores me every time it turns up.
Compared to other action platformers MM bosses just don't do it for me outside of a select few (I've had some fun blaster rucks against Gravity Man), sometimes it's because I suck at them (MM9 Wily Castle bosses pls stop abusing me) and I'm not sure I've ever enjoyed a Wily machine either. While a different kind of action platformer from the same era, Castlevania often gives me a different feeling when it comes to the bosses, one that engages more even when they're also being a bit cheesy themselves.
Unfortunately my apathy to MM bosses seems to carry over the the X series as well.