I can only speak for games I've played or attempted to play. In general, I don't have many complaints about the games, except for a couple:

FF3 (the NES copy, not the DS one) is WAY too hard toward the end. It becomes an endurance test by the end, and when it was over, I was glad to be done with it, which is not the feeling you should have when you beat a game.

FF5's story bored me, so I quit halfway. Looking back, I'd like to try again, and just focus more on the job system, which at its core is pretty cool.

FF12 didn't sit well with me at all. I really didn't like the License Grid. That thing was just lame. I also felt that the battle system became too confusing with all the gambits and techniques and things. Whatever happened to "It's your turn, would you like to attack, use magic, use an item, or run?"