MCH has the combination of being a "newer" job (people get attached to what they started with), deals ranged physical damage with a gun in a fantasy setting (people like their bows, stabbing/punching things, or exploding them), and is also saddled with the "it's really, really obvious you're under performing" stigma because MCH has a LOT of shit to keep track of constantly. The gap between baseline competence versus excellence as MCH is wider than any other class by a huge margin.

That first mark against it about being newer? Well, for one, MCH isn't much of a "set piece" class of Final Fantasy. Your only real examples of noteworthy Machinists (or relatively close equivalents) are Mustadio from FFT (which is a game that has a mixed reception and a smaller audience among Final Fantasy players) or Balthier from FFXII (which is yet another game with a mixed reception, though Balthier is probably the highlight of that game regardless of your opinion on the remainder). Gun-users are relatively uncommon in FF and the general style of the MCH playstyle doesn't lend itself well to people that are fans of, say, Vincent or Irvine. And if you want to go back to something like FF5 that had something like the Machinist in the Cannoneer...let's just say that's not much of a memorable or interesting class.

So yeah. It's one of the newer jobs of the game with relatively little lore or RP interest baked into it and it's pretty blatant if you're not playing the job as well as you could. I'd wager that the ammo system and the mechanics of Wildfire make it really, really obvious for even the most oblivious people to tell that they're doing something wrong.