What Native APIs can't be used with Capacitor&Cordova?

I know I can search the GitHub Capacitor Community plugin page or look at the Capacitor Core plugins to see what can be done.

But I’d like to have a better understanding of what CAN’T be done. Or would be hard to do. Or what can only be done with significant drawbacks?

And I know you can potentially build plugins myself.
Which I think means, that I could write in the native language and then export it as a plugin and access its functionality via API calls.
What would be drawbacks to look out for when doing that?