Never got too deep into Final Fantasy X, and havent really had a chance to go back and advance the story.

Do not understand the sphere grid at all still.
And finding the bosses decently challenging... at least so far.(Yuna had just unlocked her second Aeon last time I played it).

I am not that far in the game at all, so I can not say much, but I find it a lot harder than VIII though I understand VIII now and it is abusable.

Let's see..

VIII - First playthrough did not know what I was doing, had no clue of junctioning, and leveled up like mad... got destroyed by Ruby Dragons and T Rexes. Managed to beat the game, but was taking heavy casualties and did die a few times. Second time through, I knew about the junction system more.. Was able to have a much easier time and beat it pretty easily. Last time through.. laughing as I kill bosses in one or two rounds with my level 7-13 party in disc 2 and early 3. Junctions are the key center force of everything in this game. If you understand them and know to draw all the magic you can, equipping a magic to strengthen your mag stat so you can draw more.... and if you level your AP and not your EXP, it is a breeze.

FF1 - You can level very highly and abuse it a little, but you are very limited on healing in this game, and it is more a test of endurance and running ability to survive the 693,029 random battles before the boss who dies in 4 hits or less... and wasnt much stronger than the regular enemies. Even a lvl 50 party will eventually run out of healing and cannot live forever... unlike some other FFs... As there are so few reasonable healing sources and no mana recovery in dungeons... they are mortal.

FF2 - Have not made enough of a play through it to accurately talk.

FF3 - don't own
FF4 - see FF2
FF5 - see FF2
FF6 - Got to the last dungeon with a mid 50s party and lost my save.. havent been able to really start again or dont have the time to invest yet. You could level up, but from memory it was still a little challenging from a damage aspect, similar to FF1.
FF7 - Infinite megalixirs..... erm.. yeah. Still kinda abusable if you level real high and max alot of materia.
FF8 - Probably the most abusable of any FF game.
FF9 - Pretty easy, didn't see any way to stack the deck and make it super easy.. was reasonably hard... but also easy.
FFX - hard so far, but I haven't learned the abusable tricks yet. Difficult to understand the ability system.