FFVIII didn't really work because there was never any explanation as to who Ultimecia actually was. If that was properly explained (one theory is that she is actually Rinoa, who goes mad with grief when Squall is trapped in limbo after Ultimecia's defeat, and she finally breaks the cycle by rescuing him from it), then the game would be considerably better.
FFIX was actually really good. Half the characters are pretty interesting, and the other half have a diverse enough range of abilities that you actually want to use them in battle, even if you don't care for them. There are no gaping plot holes or things that go unexplained, save for the final boss, but by that point, everything had been wrapped up, and the final boss was probably added to give that extra bit of challenge.
Like FFIX, FFX had a cast made up of good and bad characters, but was let down by the way Tidus and Yuna were forgettable and poorly-acted. He was a spoiled, whiny brat whose development came in a sudden rush in the final cinematic, whilst she was less interesting than grass. I mostly just completed the game with Auron, Wakka, Lulu and Kimhari.
I think FFXII was let down by two things: Vaan and Penelo had no real reason for being in the story except as narrators, but the only actual narrator is Ondore. Basch were originally intended as the main characters, with Balthier as someone who was equal parts help and hinderance because of his interest in the nethicite. But then Square decided to reconsider their demographics and felt that older male characters were not interesting to most players in Japan, and worked Vaan and Penelo into what already existed. Secondly, the character progression system allowed for complete flexibility, so there was no challenge in trying to figure out which characters you needed and when. As a result, I found that I divided my characters into "Party A" and "Party B", with Party A getting all the good equipment and Party B only getting it once Party A got something new. And it didn't help that Gambits and spells only became available at certain points, much later than you would have needed them otherwise (ie you cannot set up a Gambit to target enemies that are weak to fire until Balfonheim, which is late in the game). But the challenge came back to me once I worked out my own informal character classes and limited the abilities of each.
