Probably the most useful way to get a read on this is to compare it to Titanium, also a JS-to-native framework that runs JS on a separate thread. The biggest problems with Titanium were not performance. A couple of the most common complaints:
1) You don't have full access to native SDK functionality (e.g. all the latest cool things in iOS8). You're going through a cross-platform API wrapper and limited to the choices of the framework architect. So it can be frustrating to go down this path only to find you still can't quite get the native experience you want.
2) Debugging is harder because the native toolchain (e.g. Xcode) doesn't understand the framework. You have to rely on tools provided by the framework.
AFAICS the author doesn't address these issues. He seems to focus largely on the (theoretical?) performance optimization of not crossing the JS-to-native bridge as much in React... by being even more isolated from the native APIs and doing more work in JS. But even if true, performance was not the chief complaint with the closest predecessor to this.
They are working on web-like APIs so you don't have to necessarily deal with the native platform for basic stuff like file/network access, etc. For more advanced stuff it's super simple to make your own wrapper. But yes, this is a constant downside to anything like this (only thing to compare is how difficult it is to wrap, and they aren't done with that part yet).
For #2, there's actually a great story. You can literally run the JavaScript inside an existing engine, like Chrome or Firefox, and serve it from there. You can use the normal devtools and set breakpoints on the JS.
1) You don't have full access to native SDK functionality (e.g. all the latest cool things in iOS8). You're going through a cross-platform API wrapper and limited to the choices of the framework architect. So it can be frustrating to go down this path only to find you still can't quite get the native experience you want.
2) Debugging is harder because the native toolchain (e.g. Xcode) doesn't understand the framework. You have to rely on tools provided by the framework.
AFAICS the author doesn't address these issues. He seems to focus largely on the (theoretical?) performance optimization of not crossing the JS-to-native bridge as much in React... by being even more isolated from the native APIs and doing more work in JS. But even if true, performance was not the chief complaint with the closest predecessor to this.