I'm positively surprised that Dropbox (at least from what I understood from the post) didn't require lots of changes or patches on top of the upstream codebase of Envoy to migrate their traffic!
We do have some local patches as well (mostly for integration with out own infrastructure - stats collection, some RPC specific stuff). As SaveTheRbtz mentioned we encountered some issues with non-RFC clients, corner cases which were not exposed when envoy is used in "trusted" environment, etc., but all our fixes are now in upstream, so next migrations will be way easier both for us and for other envoy users.
