The only downside to Flutter is the immense amount of effort to bootstrap an approach like that. I think it made sense for Google since they have the resources

The only downside to Flutter is the immense amount of effort to bootstrap an approach like that. I think it made sense for Google since they have the resources and free engineers to throw at the problem, but for more resource-constrained project I'm not sure it's tenable. Nevermind the fact that the ongoing maintenance burden is also a thing -- using iOS7 UI elements on iOS9 is probably a non-starter for a large part of the audience.