I've been building in Flutter for desktop (macOS) for a year and a half. It's _still_ feeling early, but if you're comfortable writing a bit of native code, it pretty much just works (for single window applications — hopefully that changes soon).
Writing code I'm fine with, it's coaxing other peoples' code to build that drives me nuts.
But Flutter didn't come out until 2017?
Not for iOS and for the public, but Google had been talking about and demoing for a long time before that.