Right now the major limitation of SVM and nodes is that you cannot express loops. Things that are easy in OSL, GLSL, or any other shader language, are very hard, or next to impossible with nodes and the SVM. Unwinding loops manually with nodes is a pain, and generally uses up all the SVM stack space for anything more than a few iterations.
Until the SVM gets loop constructs, I would consider it feature incomplete. It is unfortunate considering the speed boost you get from GPU for procedurals.