In my opinion and from our experience with +450 GDPR projects the register of processing activities forms the backbone for a good privacy accountability framework. Just sticking to the few requirements of Art 30 GDPR however is not enough. 
I will clarify what other elements can be easily added to make your register an instrument that helps you for accountability for your daily business and in case of an audit by your Supervisory Authority.
Bavo Van den Heuvel
Bavo Van den Heuvel
Partner @ Cranium