Limitations of client/events

Hi!

In the client/events documentation , there is a paragraph about profiles update limitations:

Note that to update a profile's existing identifiers (e.g., email), you must use a server-side endpoint authenticated by a private API key. Attempts to do so via client-side endpoints will return a 202, however the identifier field(s) will not be updated.

I would like to know exactly what limitation do we have on this route, because I could use it to update the phone identifier using it, for example.

Thanks