Earlier quoted context omitted.
They don't have a great story around extensions - the ones they do have are unsupported/buggy. For example, PostGIS is missing "ST_FromGeoJSON" because it was compiled with the wrong flag - and has been this way for over a year despite hundreds of user complaints.
Disclaimer: I work on GCP Can you point me to the complaints? I will take a look.
Public issue tracker link: https://issuetracker.google.com/issues/37302950