@b0rk Wrt the “force pushing makes code reviews harder”, GitHub has had links that diff between the old branch and the new one for a while (initially hidden behind the "force pushed" in the "User1 force pushed commits…" line, now there's a dedicated and prominent button on that line).
So at least it's easy to see what changed between the two PR "states"; it's much harder to see what changed in each commit though; but nothing can give it to you besides commit-oriented review tools, such as Gerrit