but it gets worse. the UI can't or won't do trivial rebases. so either you upload a new, rebased revision, and get the reviewers to re-review (because stale reviews get wiped), or just rebase, push, and mark as merged. i get you are supposed to review rebases, but this can grind a team to a halt. you're waiting for approvals, but by the time you get them someone else has pushed a new change... a colossal waste of time (edit: especially if you require 2 approvals, which i do think is good practice, and some AWS teams do it).
this also means people are trained to ignore the merge button or expect it to be greyed out. but it might be greyed out for other reason though. in GitHub, you have big green check-marks or red icons to indicate which conditions have and haven't been met (reviewers, CI, merge conflicts). in CRUX, hovering over the greyed out merge button gets you one cause at best (there is a page/tab that shows more detail, but almost nobody looks at it).
obviously it depends on your team, but it requires near constant vigilance to maintain high quality with CRUX, especially with a big team or less experienced devs contributing. process is supposed to make quality easier, not harder!
there are many other smaller issues, but that's the big one IMO. as for ReviewBoard, personally i prefer Gerrit, which itself doesn't have a great UI, but makes up for it in the workflow.
I personally love the Crux tool and some of its add ons.
As detailed, that only prevents merging via the UX, not pushes to the main branch. Maybe the team does suck, but I still consider it a shortcoming of CRUX et al.