From History it all started with a Tech Preview with react.
The goal is to support all three frameworks equally, so that implementers can make the choice of framework to use based on the needs of the project and team preference. As of tech preview 4, all frameworks have feature parity as Rich mentioned in his answer.
For a team that truly has no preference, the React ecosystem and developer base is the largest of the three supported frameworks. Until next week, anyway :)
Note: I am on the JSS team.
answered Jul 31 '18 at 15:52