I've built a custom User module in the backend similar to the Administration section but with minor differences to handle front-end registration and users separate from administrators.
I have new Member modules and Authenticators in the sapphire code to handle the new tables and module. I can login using the existing form but it kills my session immediately.
When I login using an administrator account, it recognizes that I have logged in through session. It doesn't recognize the other way though. My login will say "Welcome back," with the respective name of the user, just as it does with admin.
The only difference I'm seeing is that it doesn't maintain the Session::get("loggedInAs"); the same way.