TL;DR
Heat management is what I believe to be the biggest issue, not from it's numbers, but for accessibility to it's main gimmick: tactical, heavy burst DPS to exploit burst windows.
I've enjoyed MCH since launch, even though it's DPM was known for being abysmal in many updates, it seems SE has been at a huge disconnect for the job's balance since 4.0 and this job, to me, still feels wrong. Perhaps it's just me but I feel like this job's DPM has been significantly buffed, but it's burst DPS has been somewhat broken.
I think that when MCH got revamped for 4.0, most people greatly enjoyed the new flow and style of the job from what I saw and asked. The problem was that their burst DPS was very low and their sustained DPM was downright depressing. I'm pretty sure SE wanted this class to be a tactical burst support job, and the gameplay was just about perfect to fit that style. The problem came down to raw numbers and laughable potencies for their abilities that completely reversed the effect that was intended, such as Overheat and Overdrive.
As a result, SE buffed the potencies and fixed those two major issues, but they changed heat values to being half of what they were, essentially turning their burst DPS into an inconvenient mess to get into unless everything lines up perfectly, which in practice hardly does.
As of right now, I think MCH has statistically found it's place but the rotation is far too strict due to the forced overheat mechanic and lack of convenient heat management abilities. Our heat management was fantastic on launch due to the Cooldown not being so heavy on heat loss in comparison to how quickly you can build heat with +10 heat per shot. Cooldown only equated to 2.5 GCD's worth of heat, making it an easy ability to help manage your guage. But now it cools WAY too much heat for it to be considered as convenient since now we generate heat at half the rate (Doubling our time to build heat). Now our only convenient way to force the barrel to overheat and enter our burst phase is by using Flamethrower, which is not exactly optimal for two reasons:
1. 60 sec CD can cut too close to rotations causing overheating issues in many situations. When using Cooldown, that requires Flamethrower to do two ticks of heat (a full oGCD window) to force overheat and thus break your rotation. Otherwise, you wind up sitting and waiting, not using your GCDs until Flamethrower finishes it's CD.
2. For AoE purposes, you WANT to use Flamethrower during Overheat's buff but its too difficult to overheat without using it. Using Flamethrower is a cheap way to overheat but defeats much of it's purpose as a powerful AoE damage ability. As well as many AoE phases ending too quick, further trivializing it's use into simply being an Overheat tool.
My suggestion to fix MCH hopefully once and for all til 5.0 is to either revert the heat generation back to it's 4.0 style as +10 Heat per shot. That or adjust how Cooldown works by possibly reducing it's heat loss to -15, rather than -25, and/or possibly reversing it's effects when using ammo so as to allow rapid heat buildup to help us force overheat without wasting Flamethrower and our GCD flow. This isn't a fight for higher numbers, this is more so for returning flow and accessibility to our job.
Also if they could change Flamethrower so that we can walk while it's active, that'd be great...
Perhaps this is just a me issue? But what do current MCH players and non MCH players think? And what keeps people from playing the class so that hopefully we can shed more light on why it's so under played (From what I've seen.)