It doesn't matter much in practice - most people just use Unity or Unreal, which make it easy to target any major VR system. To the credit of these game engines

It doesn't matter much in practice - most people just use Unity or Unreal, which make it easy to target any major VR system. To the credit of these game engines, it's just not that hard to be compatible these days. That's why Oculus has started to use exclusivity contracts to gain an edge.