> For people in the Apple ecosystem, they don't want to see something that looks like it belongs on Android.
I have not seen evidence among normal people that this is true, given that the most popular apps all have custom UIs that don't necessarily follow Apple's HIG, TikTok for example. Regardless, the point is that if you're making an app for every platform like iOS, Android, Windows etc, you must make a choice lest you exclude all non-Apple users for example. ByteDance does therefore indeed use Flutter, based on this (at least in some capacity).
> mobile, tablet, and desktop experiences are often quite different
Don't conflate unified UI to mean non responsiveness. Look at Spotify, their UI is essentially the same in design except it is responsive among mobile, tablet and desktop, all while not looking native at all on any of them.
Regarding accessibility, many frameworks can be robust in that simply because they hook into the native implementation anyway, so I'm not sure where this trope of not having accessibility comes from. Here is Flutter's for example, with a dev in this thread also having talked about how they have one of the most robust implementations of accessibility that they have found among any framework: https://docs.flutter.dev/ui/accessibility-and-internationali...
It is exceedingly rare for the multiple devices to span the UI toolkits of more than one or two company-platform. For people in the Apple ecosystem, they don't want to see something that looks like it belongs on Android.
Regardless, "unified UI" doesn't mean it can't use native controls. More important from a UX perspective is that controls are in the same places and behave similarly, and that the app is organized in a similar manner, plus or minus what might naturally change because of differences in screen size and input method. And that's another useful point to make too: mobile, tablet, and desktop experiences are often quite different, and that's normal and expected at this point.
Another consideration: it's a lot more difficult to make an accessible app when you draw your own custom UI controls with your own custom behavior. If you use the native UI controls, you get a lot of the accessibility features for free, and have to do much less work to make sure everything works for people who are vision or mobility impaired. From what I understand after reading about blind people's experiences on various platforms, most companies that do custom UI don't bother, and the accessibility of their apps is atrocious.