It's missing some polish (notably, check accessibility) https://flutter.io/widgets/accessibility/ "Make your app accessible. (This is a work-in-progress.)"

I haven't dug into how much is done though. It's alpha after all, but I'd be wary of choosing it for a production project at the moment.

The hardware had a really wimpy SoC, but a native app would never chug for something as simple as pressing a button to change a number.

It made suspect that while in phones with strong GPUs the acceleration makes things buttery smooth, it doesn't graciously scale to poor GPUs (but someone please do correct me if it might have been something else)

It got me worried about shipping an app and finding out it doesn't run on low end hardware even though it doesn't do anything that would be demanding on native