It isn't proprietary, it is open source php. Not wonderful, but the cli works well and hits all of the phabricator bits you need. I've been using it for ~6 months and like it quite a bit.
Maybe proprietary isn't the right word. I don't like that the only way to do pre-commit code review is using a CLI tool that only works with Phabricator. Contrast this with the GH-style system where you use a standard Git client and a web app. The GH-style system is just much more accessible.
While GitHub isn't that great, it's sufficiently popular that there's a bunch of tools built on top of its API that supplement its failings. For example, for code reviews you can hook up https://reviewable.io (disclosure: my project), which automatically inserts links to itself into PR descriptions and doesn't require using any new CLI tools.