If you're looking for a drop-in improvement to GitHub's code review experience, you may be interested in CodeApprove (https://codeapprove.com).
It's got a lot of the things that make Gerrit appealing, but with less of a drastic workflow shift (still branch-based PRs) and a much nicer UI.
- You have to trust a random (no offense meant!) SaaS company with full access to your repositories, and to not disappear in a year or two.
- GitHub API rate limits end up causing issues sooner or later. For instance, Reviewable would randomly break and ask you to add more admin users so it could load balance API requests across multiple accounts!
- Likewise, you are still forced into the PR model and things that are trivial in Gerrit, like stacked diffs, are still hard. spr helps[1], but at that point you are piling workarounds on top of workarounds, might as well use a tool that supports the workflow natively...
- It gets messy unless 100% of the team is using it because then you have to somehow sync comments and approvals back and forth... And getting 100% of the team to use it isn't much easier than convincing them to use Gerrit, with all of the downsides.
The first two (trust with repos and GitHub API rate limits) don't really apply to CodeApprove in the same way they do to Reviewable. Because we're using GitHub's newer "apps" system and not OAuth, we only have very limited scopes (can't write code, etc.) and API rate limits go up as we get more users.
Your points about stacked diffs and PR adoption stand!