I want to start this off by reviewing Stephen Hunt’s Rise of the Iron Moon. It’s the third in a series, starting with The Court of the Air, but it stands well on its own.
It’s steampunk; that’s more or less inarguable. The question is, what makes it steampunk? It has brasstech, a more or less Victorian social and aesthetic atmosphere (complete with workhouses), and steam-powered robots. So those are more or less classic markers of the SF subgenre of steampunk.
On the other hand, it also has multiple races (including the aforementioned steam-powered robots, who are sapient and self-perpetuating), a nation state under attack by invaders, and magic – even a bloody magic sword. So that’s your “gaslamp fantasy” for you.
As far as the -punk component goes, it’s got a royal family subjugated and kept in squalor (though still Genetically Superior – less a Missing Heir rising from obscurity to save the world than a set of heirs kept around in case they were needed), a Parliament that works by violence, and a lot of blood and death.
And as far as non-Victorian SF goes, it’s also pure Dan Dare-grade docsmith stuff, with two-fisted fights in the dank, strangely twisted interior of the – well, you can fill in the details yourself. They’re all there.
So that’s a set of roots like Japanese knotweed, there. One of the fundamental problems with the classic SF movement – you know, the ultra-rationalist idea of prophesying the future, introducing a novum and extrapolating what would really happen in a world with that novum, these other three random hidden assumptions, and the rest of society staying exactly the same as it was – is, well, that it doesn’t work. What we’ve learned over decades of doing that is that doing that doesn’t bloody well work.
What does work, on the other hand, is the glamour and wonder of Science. The thrill of engineering, of invention, of delight in craft and Mastery. It may well be technologically implausible these days, but then the only useful definition of “plausible” for SF purposes is “things nobody’s yet proved won’t work”. Only the glory of engineers lives forever.
What really is implausible – what breaks our immersion, what reminds us constantly that these are historical texts and must be interpreted through a lens of their time – is the social and cultural context that these Science Heroes live in. And one of the criticisms that gets constantly levelled at steampunk is that same one – that the social and cultural context is wrong, implausible, impossible.
The criticism’s correct, of course. But it’s also missing the point, because that’s the idea. It’s not wide-eyed unicorn-spattered utopianism; it’s deliberate dissonance, it’s the invocation of a time and culture that never was, never shall be, and never should have been, in order to express those same tropes of wonder and delight. It gets the implausible cultural context out of the way to start with, in the full recognition that there’s always going to be some there, for someone, and we may as well start with one that nobody’s ever been in, and which we all know is heavily problematic, but is nevertheless familiar to everyone who’s likely to be reading it.
 That is, non-Victorian level of technology powered by Victorian means – which strictly speaking Does Not Work, and if it did would require a hell of a lot of constant intervention by a great many skilled workmen and unskilled labourers. Sigrid Ellis has a fantastic rant on that, even namechecking Bazalgette and talking about the wide base of the tech tree needed to support all of that.
 Steampunk Victoriana is full of aristocrats and wealthy industrialists, but it’s also full of street urchins, black-gang crewmen, and factory kids. This ain’t no Deco future here.
 You’d hope, anyway. But there are still some people who don’t know that “Victorian” is basically shorthand for “racist, sexist, classist, imperialist, colonialist, and practically everything else you can think of”.