I'm going to give my perspective, as some one who's leveling all classes simultaneously, but for me I honestly disliked doing dungeons as a Monk and Black Mage... I'd end up levelling them last. I'm not at max level though but for me, Greased Lightning always felt so artificial of a mechanic. You get it to full at the start of a dungeon by form spam thanks to the skill that lets you use any weaponskill, after that point, spend your time on a (arguably) strict rotation bc you need to never drop it.

On the occasion you clear a mob, you then STILL need to pay attention and use Form Shift when other classes can just do w/e in between pulls.

Things like these just made it sooo not fun. I mean it totally depends on a person but I hate rigidity when I'm playing a class and for me, Monk and Black Mage are the two that used to fit that definition the most lol. Now with the new Monk changes, it's so much more fun to play bc you're not limited by having to monitor GL anymore, lifting restrictions on what weaponskills you want to use (Which arguably isn't much bc now I just end up spamming Leaden damage with that weaponskill resitriction lift skill but w/e lol)