You didn't miss much. HW MCH was defined by Wildfire megacrits, a ridiculously complicated rotation for 15 seconds followed by 2.5 minutes of downtime. It had some of the problems SB lampshaded, but mostly it was all about getting a 25k wildfire crit. Do be aware that when MCH was doing 25k wildfire crits, a well played BLM would be pulling about 2000 DPS. It was a super big number. Also gun mage with a ridiculously dumb casting animation, but at least the class played mostly alright with cast bars, it just had a major bait and switch problem because of it.
SB kept a lot of HW's MCH, but basically focused it to a point that the class broke, while throwing in a second mechanic that the devs under-developed and then in ShB deleted outright, replacing it with a rage bar. It's a very similar problem to what happened and is still happening to Summoner in ShB. The classes were and are defined around having 2 different design directions that directly compete against each other instead of complimenting each other. What's sad is that the heat mechanic in principle in SB was actually a good mechanic, it just needed about another 6-12 months in the oven while ripping out parts of the rotation that didn't work with it, because the heat mechanic worked really well with the class thematically, offered a uniqueish playstyle not seen in other classes, and if actually utilized properly could have been really fun. Instead, it was tacked on to other mechanics and the 2 mixed like oil and water. And the mechanic that broke the cohesion? Wildfire. In today's MCH, Wildfire could basically be deleted, the 1200 potency it represents distributed elsewhere, and I doubt most players would actually care. It's been a major bane and holdback to MCH's overall design, and I say good riddance to bad rubbish at this point. That said, wildfire in HW was fun, but likewise hurt the class because so much of the classes damage was tied up in a 90s (I think? Might have been 120s) rotation. All the memes about bosses jumping with machinist happened because wildfire was too important to miss, and miss it did.
In my opinion, the shadowbringers machinist is the best machinist has been. And I actually liked, though didn't main, MCH back in HW. Granted, I had a 15ms ping instead of the 80-110ms ping I have now, but that's a topic unto itself. The big thing is that it has a coherent direction and focus. Sure, it was dumbed down, but a lot of that dumbing down was busywork that made the class especially inaccessible, as well as streamlining a really, really dumb part of the rotation.
Additionally, Machinist (still) has the best pets in the game. Namely, they're fire and forget, cannot drop GCDs unless the boss jumps, which you have a button you can push to make up some of the DPS loss from that anyways, win win win. Meanwhile, over in summoner-land......