I've been reading the paper Scalable Zero Knowledge from Cycles of Elliptic Curves, which elegantly solves (what seems to me) the main problem with zkSnarks; namely that performing non-trivial computations requires building enormous arithmetic circuits. The authors do this by showing how to compose verified computations via circuits that verify the proofs generated by other circuits.
My question is why this technique doesn't appear to be getting much use. Is the overhead from the proof-verifying circuit the main reason? I understand that vnTinyRam (the verifiable simulated CPU they introduce in the paper referenced) executes instructions very slowly, but is this because of the generality of the simulated CPU or is the overhead inherent in the PCD approach?