Steps to reproduce the issue

Set the Useractivation-Parameter to 1 or 2. When using the registration-model in your own component (for example, if you need to extend the registration for any case) you have to call:

require_once JPATH_ROOT.DS.'components'.DS.'com_users'.DS.'models'.DS.'registration.php';
$user_id = $model->register($user);

Expected result

Somehow the user-id should be retrievable. It would be ok to use:

$return = $model->register($user);
$user_id = $model->user->id;

Actual result

You can only get "useractivate" or "adminactivate" when te useractivation-parameter is set to 1 or 2.

System information (as much as possible)

Joomla 3.4.8 and above

Additional comments

Could easily be solved: in Line 624 of the registration-Model: Change this

    if ($useractivation == 1)
    {
        return "useractivate";
    }

to this:

    $this->user = $user; // make the user-object available in the object for later use.
    if ($useractivation == 1)
    {
        return "useractivate";
    }