junseth just criticized ring signatures on the Bitcoin Uncensored podcast, saying that Gregory Maxwell knows more about how this stuff works than the Monero devs. 
Starts around the 39:54 mark: https://soundcloud.com/bitcoinuncensored/e8-the-refuge-of-scoundrels-092615
Starts around the 39:54 mark: https://soundcloud.com/bitcoinuncensored/e8-the-refuge-of-scoundrels-092615
When did Gregory Maxwell say "Monero won't work/won't scale"...?
He has said that it isn't compatible with the sort of UXTO pruning that Bitcoin can do, which is somewhat true (there are indeed a few tricky ways to make it work if you really want to). I've seen this on reddit and possibly elsewhere. It is also a bit disingenuous since he has also said that despite UXTO pruning, Bitcoin also can't usefully scale without going offchain, which is why he opposes increasing Bitcoin's block size too much beyond the current 5-7 tps). In fact, if I'm not mistaken he may have suggested lowering the block size, but I'm not positive of that.
I don't think he has ever said Monero "won't work".
UXTO pruning provides only a linear advantage over no pruning while the blockchain growth until the currency, be it Monero or Bitcoin, becomes close to mainstream is, to a good approximation, exponential. Scaling basically depends on a continuation of Moore's, Kryder's, and Nielsen's Laws at an exponential rate of growth greater than the exponential rate of growth of the blockchain. Monero has the ability to scale with Moore's, Kryder's, and Nielsen's Laws.
