bosses have no levels so no way to estimate power at a glance
no way to view how much HP the bosses have
most of them are comically OP when you first encounter them, with a single hit of a multi-hit combo one-shotting you, while they can facetank dozens of turns of your attacks before falling
your options are either play on babymode retard difficulty, play dozens of rounds flawlessly never taking a single hit, or come back to them later (with most people choosing this latter option)
this means you have to individually remember each boss and backtrack all the way to it
you must carefully maintain your power level, because if you are even slightly overleveled, you will just absolutely steamroll the boss due to how exponential power gain is in this game
Conceptually the boss fights would be fun, but in execution it always feels very one-sided. Why the hell would you put all this effort into learning a boss flawless when you could just come back and mop the floor with them after some level ups?