Clicking is bad in alot of ways:
1# it takes longer to reach a skill with your mouse than using a button, this is really bad when you have to reach an ability quickly, like a dash. (try knockback canceling by clicking,)
2# it is distracting, because you have to look at your curser and where to click, while looking at your curser, you can't look at the fight.
3# you can not move with your mouse while clicking, this is escpecialy bad in FFXIV because your character turns to the boss when using skills.
I have had people in my statics that were raiding with a controller, because they played on PS4, it works quite well, but takes a lot of time to get used to i guess.
But clickers are at the most disadvantage of all three types.