Openflow and SDN is in no way different from other control mechanisms for network equipment. It's up to the administrator of a network how he decides to secure the equipment. The connection between the Openflow capable switch and the controller can run over TLS, and can even be authenticated by SSL certificates. Thats gives you the same level of security as you have with e.g. key based SSH connections. From this perspective the level of security is comparable.
But keep in mind that if the trust relationship between the controller and the switch is established, the control capabilities the controller has will be much broader and more granular than a manual or automatic configuration of existing switch features can be. Therefore there is some room for bugs and failures in your SDN application.