FFXIII did at least prevent overleveling or overpowering your party before the time for that came.
All of the previous FF's can be broken due to that.
FFVII - Materia system is broken as fuck. Early Beta, Limit breaks etc can kill any difficulty the game has.
FFVIII - Junction system is broken. You can get very good playing cards for Triple Triad (Zell, Seifer, Diabolos, Qusitis and Ifrit) right near the start of the game making you unbeatable at cards which you can then use to get Lion Heart early, uber spells early. Broken game.
FFIX - Not a hard game, The only challenge comes from trying to steal rare items while bosses are kicking your ass. If you just fight the game is easy and just to be extra sure the infinite money trick with Cotton Robes can be done early in disk 1 giving you power items and abilities early and then the Chocographs are there to take the piss on the difficulty some more.
FFX - Game so fucking easy. Main game is a joke with rinse and repeat of slightly more powerful enemies fought the same way for majority of the game. Don't need strategy for the most part in the main game. Just use the right character against certain enemies for guaranteed win, which the game shows you to do anyway and for the lols the summons are way too strong.
FFXII - LOL, My favorite FF game but you can rip the difficulty out of this game right at the start. No need to even go into this, just totally broken. The game is so broken that you can set up your gambits making you not even need to touch the controller during boss fights...
Some of the boss fights in FFXIII handed me my ass it was refreshing since not since FFV did I have as many game overs as I did with FFXIII. FFV was miles harder but FFXIII was a step in the right direction difficulty wise, imo. Sadly other areas of the game were sorely lacking.