The lock-in is very bad, especially in the auth and user management but migrating out of the JSON-storage isn't easy either. It also encourages bad patterns, like developing stuff straight into cloud functions rather than locally with version control.
Personally I prefer SQL-RDBMS-style data storage in part because it has these algebraic qualities that makes transitions between 'schema'-versions fairly robust and predictable. Extending and transforming JSON-storage is in my experience rather brittle, even if we ignore the hurdles and indirections around export and import in the clown solutions.
You can go with firebase for version N of your application when you have all relations between entities defined. If version N+1 redefines what your documents are, you're looking to write a migration script. That's not so different from SQL with all the joins. But there are people who can't design a data schema for neither a SQL database nor a document store.
Personally I prefer SQL-RDBMS-style data storage in part because it has these algebraic qualities that makes transitions between 'schema'-versions fairly robust and predictable. Extending and transforming JSON-storage is in my experience rather brittle, even if we ignore the hurdles and indirections around export and import in the clown solutions.