[I play in Reboot so keep that in mind for any damage numbers]
With the amount of upgrades that are possible, it's become relatively easy for someone to hit cap damage. (10 billion) This becomes especially true in endgame boss parties with the amount of buffs and party buffs. And this causes a problem for slower, harder hitting classes when doing hard Lucid.
Hard Lucid has 45 Trillion HP, or 40.5 Trillion with the upcoming nerf. This means that over the 40 seconds you can damage Lucid, each person has to deal around 7 Trillion damage in order to do their portion. At the current cap of 10 billion damage, that means one has to hit cap damage 700 times in order to contribute. Now, this isn't a problem for many classes, as they deal far more than 700 hits in the allotted time. However, there are some classes like the one I play, Shadower, that quite literally cannot do their portion of damage.
Shadower's main bossing skill, Assassinate, does around 470 hits at max attack speed. Shadowers have a few other skills to supplement this damage, but they are relatively insignificant. In total, even if a Shadower is hitting cap damage with every single Assassinate hit and is using their other skills, they still fall at least a trillion or two damage short of what is required. Assassinate could be hitting several time the damage cap, perhaps upward of 30 to 40 billion damage on the strong hits, but is limited to only 10 billion. If the damage cap were increased to allow slower hitting classes to hit their full amount, then every class would be on an even playing field, unlike now.
Obviously, there is the option of doing less damage and letting party members cover for whatever damage you can't deal. However, if someone is over-geared for Hard Lucid, and still can't even do their portion of damage, then there is a problem with the mechanics of the game.
tl;dr Slower hitting classes suck at Hard Lucid phase 3, and so increase the damage cap.