yes, confirmed, I spend many times and now installing Big Sur. Cuz first time Im update from Big Sure, and possible load into safe mode. Yep installed on external SSD, Inside still Mojave on Fusion in iMac2019 27)
So, after upgrade I get bootloop, when attached on loading in safe mode or from 10.14 I saw in logs issue about hardwares, and error about not possible Flash from /Volumes/Update my External SSD.
Next what I do, I was try erase my external SSD, Install clean Monterey - and Its installed - data migrate from HD, all ok.
Working success 4 days, and today system stop, reboot, and again bootloop. reinstalled from recovery, not helps.
So never Im ain't will be use so fresh untested OS.
Right now repeat again all, clean external ssd, install big sure, migration data - success.