I built a couple of games with online login/accounts and if you don't have much experience with this prepare yourself for some significant headache.
Fortunately the backend-as-a-service solutions normally handle various aspects of authentication security for you.
That said, for a recent project (not in C3 though) I got tired of managing user logic and rolled back to Apple Game Center/Google Play for leaderboards etc; and local storage for any data. Way simpler and more manageable. Additionally if you plan on releasing to the Appstore you need to account for other stuff, like app rejection based on required login.
All in all I'd stay clear of this unless really needed.