But merge queues (talking in general, IDK about the mergify.com product specifically) don't require fast-forwarding as far as the developer is aware. In the simplest case it looks like merging (non-fast-forward) to a temporary branch, then only updating the main branch after tests pass. This is very similar to your auto-revert except the main branch is never broken, so no wasted developer time and confusion when they pull a bad commit to start their PR.
IMHO it is a real shame that all CI doesn't work like this. It should be the default. Just this basic delay and auto-revert is already a nice boost to developer productivity. Not to mention that blocking a merge in the original PR is much less confusing than reverting and requiring a fresh PR to make the change. It adds basically no complexity other than the fact that our tools aren't set up to work this way by default which ends up requiring extra tools which are not as well integrated.
On top of this you can add batching which can be incredibly useful when your CI is slow (including things like deploying to a staging environment and letting it soak for a few hours) which isn't feasible to do per-PR even for fairly small teams.