This is the essence of the problem - it's TOO cutting edge. Even if you can get past the accumulator problem (which seems, at this stage, quite difficult if not improbable) you have an interesting issue: because the blockchain reveals absolutely nothing, a bug in the extremely new and untested cryptography could allow an attacker to create new coins out of the blue (which is not impossible, it already happened with Bitcoin and they had to rollback the chain pretty much). The difference is that with Zerocash you'd be completely unable to tell this had occurred because of the zero-knowledge blockchain.

To steal a phrase from that CryptoNote busting post the other day, Zerocash is a "one trick pony" (pun not intended) - it offers a single, terribly amazing, terribly dangerous, and terribly unproven feature. If, by some chance, Monero is the dominant private cryptocurrency by the time ZeroCash is ready, will that feature be compelling enough to make ordinary users switch? Or will they be just fine with the privacy Monero provides? Monero will have a hard enough time catching up to Bitcoin, and I honestly can't imagine Zerocash's single feature being sufficiently better than Monero's to provide tangible value to the general populace.