I guess the way it's set up it's easier for you to enter another character's part of the grid from the start, but how the abilities are rearranged makes it harder - I found a picture of it and Ultima is hidden away in a corner completely surrounded by lvl4 key spheres so that you need 4 to unlock it, rather than 3. Plus there's just fewer HP, MP, etc. powerups on it. I suppose that could make it a bit harder until you've gotten a good way through the game.
And go read up on Dark Aeons and the hidden boss unlocked once you defeat them all. I don't think even the monster arena hybrids compare to them...which really makes me want to try and fight them.
Omega Ruins is a good wake up call, I find...you go there straight after defeating Yunalesca and you'll stand no chance 😂 I had my entire party wiped out almost instantly. I think the only actual boss I ever had any trouble with in that game is one of Seymour's incarnations, second one I think. Though I suppose the last bosses would have been harder had I not spent so much time inside Omega Ruins...