A hard look at the Two Gauges
Greased Lightning:
There is no reason to not have max stacks, I get that it's like an engine that is slowly revving up to big damage. But it's like a lightswitch, there's no reason not to have max unless you a) died, or b) messed up. This makes GL much like protect, it's a lightswitch, you have it or you don't.
Greased Lightning has always been sort of a thing since ARR which has made the job a blessing and a curse. It feels awful when you make a mistake, because you're worried you might lose this, or have to burn other things like perfect balance to maintain it. On one hand it can be satisfying to build these stacks up and keep them up, there are no more barriers to retaining stacks (heck there are 3 abilities to maintain it, including RoE).
I don't think there is very much inherent rewards that aren't passive with MNK built in with GL. They obviously don't feel the need to add in many moves that use it (with the exception of Tornado Kick). There's no big reward for dumping your stacks, nor having max. I admit it does feel fun to run around your target and hit it with faster skill speeds from GL4 (but this is partially mitigated due to limitations of gear and skill speed due to stealth nerfs because of TP removal). It may be in the future they could implement some sort of system where they change greased lightning intentionally so you have to balance stacks or use skills to drop individual stacks of GL. Maybe they could design TK in such a way that it only removes like 1 stack of GL but has some cost? or make bigger dmg abilities that drop GL stacks but make it fun to build again?
Chakra Gauge:
-Chakra is luck based (get a crit to get a 70% chance to get a stack), Monks traded damaging oGCD's in favor of luck based damage.
-The only control you have over chakra generation is manually using them, which outside of the opener or short downtimes, while rolling GCD's during uptime on a target is not a managable resource.
-It suffers from the problem of, once you get 5, use it.
-They added in another AoE skill which admittedly is flashy, but I think this was in lieu of knowing what to actually do with another gauge for MNK.
The Chakra gauge when added into HW, always felt like it was tacked on. It gives a damaging oGCD skill yes, but it never added much to the core gameplay of MNK. It did give you something to do outside of combat, or during untargettable boss phases, but it largely has never been something that has anything that evolved the MNK as a job as a whole. Also with the removal of oGCD's you're not making a conscious decision in when to use your skills.
There's a lot more I could write, but I think 2 gauges is enough.