The main disadvantage of the 'meta-coin' approach is that there can't be any SPV clients (someone needs to store the whole blockchain, as it were), but there are plenty of ways around that.
OK I don't understand the behind-the-scene details of SPV clients other than the fact that it does not do its own verification but connects to a trusted node. I feel like I am missing something here. Why couldn't a thin client do the same with XCP.