Vue 3 is going to be awesome. The only thing missing with Vue is the lack of a vue-native ecosystem.
What are the downsides that holds back on the adoption ?
That said, "doing something that other things do" is a really hard place to be, doubly so with developer tools. A lot of time and money is invested in dev environments for frameworks, and it's an uphill battle to adopt anything new once you have your build/debug/profile/etc tooling off the ground. Even if your solution is better.
And it doesn't appear to support desktop? That's really what would sway me to use this.
Sidenote: holy shit, that playground workflow is impressive.