Before this begins, I don't' think this is a flaw that seriously hurts the game enough to be a huge point against it, but it is enough to become kind of frustrating to a certain degree. I love Final Fantasy Tactics for the sheer volume of ways you can customize your characters, and the great fights the game sets up. That said, there is something about the unique characters with their unique classes that have always bothered me.
By the time you start getting access to them, you've already put a fair bit of time into your generic NPC's, and the characters with unique jobs come with pretty much nothing. This means you'll need to put in all the work you did on your other characters into your new-found PC's. And the big thing to know is that raising up your characters in Tactics can be a bit time consuming, especially if you are going in without a guide. So many abilities and class combinations, paired with some early game fight that kind of push you to develop your generic units, such as Dorter Trade City.
While some characters don't take too long to join, such as Agrias and Mustadio, others don't come until a fair bit later in the game, the Heaven and Hell Knights for example. Especially if you want to make use of those two (Or worse, any late-game character) you are going to have a long time to spend grinding out classes on them in order to get new abilities so you can customize them how you'd like.
Tactics remain one of the greatest Strategy RPG's for good reason, but this is one thing that never sat well with me. Even though I understand why a lot of people dislike the aforementioned Heaven and Hell Knights because their abilities are pretty bad in practice, I always really liked how they worked and thought they were a lot of fun, but with how much work so many others in my party had put into them, it became little more than frustration to take the time necessary to start learning all the abilities that I'd already gained with my other character's, and needlessly pads out the length of the game.