" it's cross-platform"

Nothing in real time graphics _actually_ is cross platform. You are always programming a vendor hardware.

"Cross platform" really has no value in graphics programming, honestly, when discussing the lowest level programmable api.

You always need to explicitly say which platforms you target, and then test on those platforms.

The main questions should be "on which platform will my users run it" and "can I debug and test this".

So if you are starting out - pick out the easiest api for you. Don't go looking for universality as a value on it's owm (if you want universal rendering, choose CPU rendering. if you want high perf - choose your GPU platform(s) explicitly).