I updated my RPI4B w/SSD yesterday and generally went fine (logs complained about a few things in config but nothing that got in the way of coming up). I would still suggest waiting as my HA locked up about 1:30am and was totally unresponsive. As it’s on a top shelf, couldn’t plug into keyboard and monitor to check locally so powered down and restarted. It felt very warm to the touch and when it did come back up, history for CPU temp was blank from 1:30am till 7am when I restarted, then started out at about 150 deg F, finally settling back to about 120 deg F within about an hour.
I suspect my issue is actually unrelated to 2021.9, or at least was something already in play that was exacerbated by the update, since swap memory had climbed daily starting at midnight every day the last 3 days and was up around 75% when the system crashed, is now back down at 2%.
Next time, I’ll wait for second release of the month to be on safe side, particularly after seeing issues people with RPI3 are having.