I definitely appreciate a spectrum of difficulty in a game’s boss fights, assuming the difficulty comes from actual mechanics and not just inflated stats. Failure isn’t pleasant in the moment, but it presents a chance to understand the game more deeply as you evaluate what’s gone wrong in each previous attempt. There’s huge satisfaction in seeing yourself get closer with each rematch and then eventually triumphing. Hard boss fights are some of the classical moments which allow gamers to not simply play a game, but to master it. That said, it’s nice when the harder bosses in a game are the ones which are actually important to the story, and not the random big monsters which happen to crop up in between plot points. I’ve played some games where certain major villain fights are designed around a single unusual gimmick, and the boss barely puts up a fight if the player is prepared for it or figures it out quickly. But then later the player may need multiple attempts to take down the less important villains or the generic mech/golem/whatever because the developer built them like actual boss fights instead of trying to be cute with off-the-wall abilities. You shouldn’t have to guess whether players will take your big bads seriously and feel the drama of those encounters, so if you’ve got a particularly experimental mechanic intended to raise a boss’s difficulty, maybe resist the urge to put it in a boss fight which holds a lot of narrative weight.