In my extremely biased position (as the maintainer of PouchDB) I would really have liked to still see this based on the CouchDB protocol / PouchDB codebase, it is true that neither Couch or PouchDB have particularly expressive and powerful permission systems (which is what the main downside looks to be) however I have been very aware of that building PouchDB and have made a lot of effort to ensure a powerful and flexible permissions system can be built on top of it (for example https://github.com/thaliproject/node_acl_pouchdb)
I do think that when you are tasked with providing cloud services for existing Mozilla products that there is a lot more risk involved in leveraging an existing established ecosystem like Couch / PouchDB however so its fairly likely that for Mozilla services this was the best choice (the same in reverse is true however also when looking at this as a generic library).
As for a disclaimer, I am a maintainer of PouchDB and a Mozilla developer. I have worked / talked to this team about Kinto, work on some features that use it and they are doing awesome stuff, some of which Pouch may possibly look to do as well.
