Look up the Meiji Restoration in Japan. They went from a feudal, near-medieval society to an industrial society between 1870 and 1920, by the time they were done they are participating in both world wars.

You actually don’t have to suspend your disbelief so hard here, it’s the most believable part of the story of people who can bend elements with tai chi

I think that most of the criticisms directed at the industrialization and mecha stuff is mostly just a byproduct of the worldbuilding in korra broadly not being very good. Not even necessarily bad a lot of the time, but just not as good as avatar.

Bending styles became more homogenized and choreography is worse, everything became a kind of 1920’s hong kong steampunk, and lots of the city shots, there’s basically nobody walking around. You have things like tasers and huge mechas, but huge mechas and tasers without explanations for how they’re getting such dense energy storage, or circumventing some real world problems with those technologies in a 1920’s context. With various forms of bending, you can kind of get around the energy density problem a little bit, since it’s just straight up magic that seems like it violates conservation of energy, but with korra’s stuff, that doesn’t apply a lot of the time.

Lots of little things like that kinda give the impression that the world is made of paper mache, or that a lot of things are just kind of done because they’re a cool idea, rather than because they’re both a cool idea and make a little bit of sense. I’m not really opposed to the idea of a car in the setting, but it strikes me as quite a bit easier to power a car if you have a mobile human power plant that can produce large amounts of energy. I think it’s also kind of a shame to disconnect the tech from this for a different reason, as well, and that’s because it means that the bending is kind of, less broadly useful and applicable. It takes up less space in the setting, it has less utility, it’s not as cool, and the show doesn’t really end up giving many good replacements for it as time goes on.

That’s really nitpicking, though. I think the broader point is just that there wasn’t much done in the series to really show the continuity between ATLA and korra (do not mistake this for fanservice), and they really feel like different shows. Feels kind of like about a quarter of the reason why people didn’t like the last jedi, but that’s kind of a whole other deal. Anyways, that being the case, korra’s more of a stand alone kind of deal, and I think it kind of falls flat on it’s own, because it just isn’t very good and I don’t like it as much as the OG.

You also get a lot of people that will blame all the problems on the show that it kept getting renewed season after season without any real knowledge of the future viability of the show, but I think I would still just blame the writing, in that respect. You can make a good, contiguous series of media based only on good improv, only on good setup and payoff, external to the idea that the show has multiple locked-in seasons. I don’t think it matters too much, if you’re good enough. Main example there is probably just venture bros, though.