Four words, Ruby, Emerald, Omega, Ozma. They all made the end bosses look like pushovers.
The real challenge of FFX was endurance. Just how many times can you put up with the Cloisters to get extra items before you smash the screen? How long can you press the same button repeatedly while paying stupid amounts of attention to flashes because missing a beat screws you? Are you willing to play over ten whole minutes of Blitzball over and over and over with only one really useful reward? Can you really be bothered to catch ten of every monster in the game even though some refuse to turn up and are frequently instead replaced by Magic Pots (the enemies ever)?