Codeigniter, is Session class safe?

Codeigniter has its own class for Session, that is, it does not use the native Sessions of PHP.

Note: The Session class does not use native PHP sessions. It generates its own session data, offering more flexibility for developers.

You can set a session as follows:

$data_session = array(
                              'id_login'                    => $id_user,
                              'nome'                         => $nome,
                              'userPermission'                   => 1       
                              );
$this->session->set_userdata($data_session);

My question is:

Is it really safe to use this class to write ID's that should be secret?

Is there any contraindication in which we should not use this class?