If your main class is Black Mage, then you may find this post to be either an insult to your favorite class, successful in proving my own stupidity, or both. However I want to make it clear I am not implying that the class is broken or unenjoyable for others, I personally just can't use it and highly regret choosing it as my starting class.
Reason 1: Unfriendly to lag - As a black mage, or any castor class (in-theory), the only cast-canceling status more detrimental then Silence and Paralyze is a sketchy connection. My dps will severely suffer when participating in a crowded FATE. This is because my character is actually executing the skill long before the cooldown on my screen is finished, often resulting in recasting to fail, sometimes twice in a row. Because of this, I may even lose the chance of switching from umbral ice stacks to astral fire or vice versa.
Reason 2: Situational Skills - In example, Freeze. Despite looking through different forums and black mage guides it doesn't make sense to me. There are other skills similar to that; skills that should be used in special situations, and proper execution of these skills would, in theory, paint someone as a viable black mage user. I have a hunch that such skills would only be used in end-game, which, for me, is a terribly far journey (I'm also a bit jealous of players who have 3-4 levelcap classes already).
Reason 3: Sleeping enemies don't stand out - Needless to say, sleep is a very powerful skill. However, I don't know how to stop people from waking enemies up. I guess its important for tanks to use aoe to pull groups of mobs, but when the pull fails and the healer dies, we all die. The effective time for sleep gets cut in half everytime it gets used on the same enemy, and therefore it becomes a moot effort trying to recast sleep when the healer is getting attacked by enemies that are separated from the pull. Then again, this could be a result of a lack of coordination, but I've never been a tactical person.
Reason 4: Shares the same close range weakness as healers - In situations where tanks aren't available, such as underpopulated fates, I often get destroyed by bosses since I cannot move whilst attacking. I often scream in silence when I see dragoons and monks mocking bosses when they just laugh in their face as they swipe them while skipping merrily away from a massive area attack. Correct me if I'm wrong, but doesn't the closer range dps classes do more damage then the castor dps types?
Reason 5: AoE Attacks are not meant for solo play - If you do not have at least 2 tanks in the area or in your party, area of effect attacks is the worst thing you can do as a black mage since you cannot move while attacking, and higher level mobs are capable of canceling your casting. I think this class has the best AoE ability (not sure), but at any rate its not the smartest thing to do by yourself, at least, in my experience.
I'm pretty sure that with the combination of the high black mage population and the general amount of discussions of the class, a lot of people do in-fact enjoy using it. And I can see why, the class is well designed and highly dynamic. I just don't recommend it for stupid people, such as myself.