The thing is, when you're shopping for GUI toolkits, "try it and find out" isn't a great decisionmaking rubric. Partially because there are a zillion of them and you simply can't try them all, and partially because learning one is expensive and time-consuming, so, unless you genuinely enjoy that kind of thing, you kind of want to pick one in one shot.

Meaning that, in a slate of options where one of them can definitely do something you need, and another one where the official story seems to be ¯\_(ツ)_/¯ and the examples you've seen seem to indicate that it's at least something you need to go out of your way to accomplish, there really isn't any injustice in deciding to expend no further mental energy on the latter.

That said, if you want spend your own time digging in and finding out, be my guest.

Flutter is not competing with general GUI toolkits, it's competing with fully crossplatform toolkits, of which there's only a couple of real contenders for real-world apps: React Native and maybe Sciter.

Nothing else is production-ready and can run on the web, iOS, Android and desktop OSs (Linux, Windows, MacOS).

It's not competing with pure web frameworks. For this reason, yes, I think that if you're going to criticize it, you absolutely need to know what you're talking about... if Flutter claims it can handle accessibility, it's on you to prove it can't if you make this argument so eloquently on the Internet without actually knowing it.

BTW if you know of any other good UI Toolkits that really can run anywhere like Flutter, using the same code base, please let me know.