Note only if they try to be, not if the boss just plain isn't overdone.
So final bosses in games tend to be very big events as the years have gone past growing more and more grand and awe inspired when made well, and for want of a better word, epic. Take Jubileus as a prime example where despite having some repetitiveness comes off as simply amazing and well worth the position of last boss.
But which games try to emulate such feats, making bosses in similar ways to such successful ones but in the end failing to deliver, but at the same time being clear that it did fail and not that they simply didn't try to make such a thing.
I can't think of too many myself, which was obviously why I made this thread to interest me, but there are a few I could point out.
The first being Deathwing in World of Warcraft, which sounded amazing to start with, a multiple fight boss each with multiple phases which made it sound like it would truly be an RPG style final boss. However what resulted was only 2 fights, the first of which was riding on his back plying up armour, the second of which was destroying his malforming body in a mealstrom before he could destroy the world. It SOUNDED great, but in action was just an incredibly repetitive series of fights which all the while seemed like it should have been amazing but just wasn't, I personally enjoyed the final boss but that is due to me loving repetitive things.
The second would be The Dark Sovereign in Heroes of Might and Magic 5. The game itself was very enjoyable and the final level looked to be incredibly fun and interesting, you were given all the main heroes from each campaign (save for the dead or otherwise unavailable ones) and each came with a HUGE army, all under your control and a quick march from the final boss. Sadly all it resulted in was facing against 3 bosses in a row which all had summoned units, which in this game disappear after a fight and will also disappear if all the actual units a hero owns are killed where no matter how many summoned things you have they will go and you will lose / win. So effectively to begin with this already destroyed any great feeling that existed in killing the huge armies of the final level since you were fighting against only fake units. This was further amplified by the worst thing of all, the fact you needed to fight all 3 final bosses with all 4 heroes in a row, not together in a good way, but in a row. So you had 12 fights and after each hero won theirs the boss instantly came back with absolutely no damage to them where they just start summoning in more dudes as if you did NOTHING to them. After this happening 12 times it pretty much left me feeling unsatisfied at the end, added to the stereotypical and obvious cliffhanger ending.
And then I ran out of memories of games with such bosses in them, so now it is your turn to entertain me. Do it.