Ring Confidential Transactions built on top of Blockstream's Confidential Transactions (or my roughly equivalent but apparently more efficient Zero Knowledge Transactions built on top of Denis' Compact Confidential Transactions) does not absolutely prevent undetectable abuse of the money supply.

The relevant (to your stated concern) distinction from Zerocash (and a friendly reminder to not conflate Zerocoin with Zerocash because the former requires equal revealed values and doesn't integrate with hiding values) is that there isn't a global trusted master key (generated once at setup of the sytem) to be potentially abused (if the trusted setup was gamed some how). Yet in both systems, if you can muster enough computing resources even just once (and/or break/weaken the number-theoretic cryptographic assumptions security), you can create unlimited money out-of-thin-air and this can't be detected (unless detection means everyone has the same level of breakage capability and all values can be globally unmasked rendering value hiding useless).

Homomorphic values and ring signatures come with potentially huge anti-DDoS costs as I have been explaining in a thread I started in the Bitcoin Discussion forum. In that thread, I have alluded to we might be better off to just eliminate homomorphic (hiding) values and also eliminate Cryptonote's one-time ring signatures and move to something like CoinShuffle, because we are going to need to do a CoinShuffle any way. The details on this tradeoff need to be further mulled over and elucidated.

So in summary, it is possible that Monero and Cryptonote (including Confident Transactions and the attempts to combine them with one-time rings) is one grand waste of time and effort, but that determination is not yet entirely clear to me. I need to spend some time writing down all the details so I can convince myself what are key determinants on this issue. It is possible that the conclusion may be a multifurcation.

Anonymity is very difficult to accomplish holistically especially at-scale (Monero is no where near accomplishing that at-scale) and it doesn't come for free.