Critical hits and Direct Hits can multiply (Direct Crits) which currently do ~2x the damage a normal version of that attack would do.
Typically over the course of a fight you do many many attacks and so you will get more or less averaged out critical and direct hit luck, and generally if you have more crit stat you will crit more and so forth, so a few direct criticals here and there don't really make a huge difference.
Many players want to play as well as they can and do the most damage so they can beat the enrage of fights (especially when challenging savage and ultimate fights).
Many jobs build up to or have long cooldown high-potency attacks that constitute a lot of their damage (Think Communio, Scorch, Hyosho Ranryu, etc.) Using these large attacks properly feels good and having them direct critical hit is sure to put a smile on their player's face. However given that in a typical fight a great player might use such skills every chance they can and maximize their uses, a lucky player who happens to direct critical one or two of them could end up doing more damage by a large amount even if they did a poor rotation and weren't able to use the powerful skills as often.
A direct critical hit can do more than double what the normal version of the attack does. When a player who forgets to use their most important skill once but got lucky with a direct critical can do more damage than someone who is doing everything to maximize their firepower it feels less rewarding to play well.
I wonder if very high potency attacks should perform similarly to moves like Warrior's Inner Chaos (which always direct-critical hits) or Machinist's Wildfire (which can never direct-critical hit). I believe that these skills were changed specifically to avoid this same problem.
There are obviously some concerns to consider such as the interactions with party buffs like chain stratagem, battle voice, and battle litany and adjustments would have to be made to jobs impacted.
My concern isn't with the existence of randomness, critical hits, etc. as I believe that critical hits are fun and add some excitement, especially on a tight enrage. My concern is due to Direct-Crits being worth 2x or more of a regular use of a very high potency skill.