alt Hacker News"flashing" a phone is largely the same as any OTA update. There's of course always a risk of it going wrong, disk failures are always possible, but it's exceptionally hard to do so accidentally. Especially with custom ROMs where they basically never include a new bootloader, so "flashing" is no different than installing an OS on a desktop system - it's just writing to the boot partition. Which you can always do again since the bootloader is still available.
It is not 'largely the same as OTA' on phones with downgrade protection. Once you lock the device again, it's game over because the bootloader refuses to boot an older version of the OS, and you cannot unlock the phone anymore. Happens all the time in the /e/OS and Fairphone forums.
It really depends on the device. E.g. Pixel is quite hard to brick. Though they do sometimes increment the anti-rollback version:
https://developers.google.com/android/images
In that case you have to be careful to not flash an older version to both slots and lock the bootloader, which is possible, because many non-Google/GrapheneOS images are often behind on security updates.