I think, to generalise a bit, you should probably work on your hardest problem needed to launch. If that's an awesome UX, you should probably make sure that's p

I think, to generalise a bit, you should probably work on your hardest problem needed to launch. If that's an awesome UX, you should probably make sure that's possible, and especially with front-end stuff validate that it's usable. If your backend has unsolved problems that are needed for launch, you probably want to make sure they're solvable before you spend ages implementing the frontend.