Clearly the Qur'an isn't a self sufficient text, no text is regardless of what it might state or what people might like to believe.
The problem with most Quranists in my experience is they tend towards a rather undeveloped and naive hermeneutics.
The lack of a sophisticated hermeneutic is also met with a lack of an ability to explain when and how everything went wrong within the Muslim community. The Christian parallel would be the doctrine of the great apostacy held by some protestants.
Aside from Joseph Islam, I've yet to run into many Quranists online with well developed critiques of mainstream Islam's approach to hermeneutics. Further rather than merely offering a negative critique of thorny issues, Joseph Islam provides a productive and positive account of to understand the many topics he has touched on.
I think more work like Joseph's is required online.
What I am interested in seeing is a more maximalist Quranist approach rather than the reductive tact often taken which ends up sounding not unlike some contemporary new age cliche. Or in other words a more radical and barren Orientalist approach revisted.
https://www.facebook.com/QURANISTHETRUTH/photos/a.208888389278682.1073741826.208884229279098/474478176053034/?type=1