I discussed this point in a different thread but it would be worth discussing it separately. I understand that upsource does not provide the flexibility to automatically create reviews on every commit. That has its own downsides for manual follow ups, missed reviews etc. It would be great if you could reconsider this or as an alternative automatically create reviews by parsing git commit messages for e.g. look for #reviewthis (like github does) and #attach123 to attach the current git commit to the review 123.
Note reviewboard does support automatic review creation and parsing git commit messages. Looks like a good feature to have, doesn't it?