MCH's problem isn't its damage output, it's the highest in the game. It works at low ranks, but as soon as one reach crystal and especially Omega+ games, people know how to play around it (notably wildfire or just playing around the turret), it has no mobility, no survivability and dies to anything that looks in its general direction. By this virtue it also makes the job literally unable to rotate on crystal well or assist in pushing because positioning matters so much. It's a bloated kit against low ranks that peters out past Diamond level. It doesn't mean it's unplayable above, in fact some people do have good success with it, but it's still a low tier job there.
Just to quote SE with the latest changes: Machinist win rates are unexpectedly low, and we found room for improvement in terms of their durability, so alongside adjusting maximum HP, we increased the potency of Scattergun to improve machinist's ability to handle incoming foes.
I don't know why the "unexpectedly" because we've been literally complaining about it for ages. The HP increase is something at least, but they otherwise keep increasing the job's damage output which is in my opinion only making the job even more lopsided. MCH can be incredibly oppressive when used right, but it's still struggling to not grief its own team at the same time.