Experiences after installing Android 8 Beta4 FW available in various formats
- update.ATV8to8-U5MINI only-normal-190904-warning-This FW does not support rollback.zip
- update-atv7gotoatv8-U5MINI full-190904 This FW does not support rollback.zip
- Update via ROM emergency recovery (pressing OK button on remote control during boot)
https://mega.nz/#!y8MxEKKC!lsTUGQDLrn6MvZNrHRKo19wgiy6w_j8bRQJdA79OqcQ
This is a later ATV7gotoATV8 image. I can't be used for a mere ATV8 update.

With all images the bootloader can be used to directly copy that image to flash memory.
Press 2x OK quickly on the remote when the Google menu just appears for that.
Updt will appear on the front display.
Indeed a rollback option is not included but using the most recent version :
- The boot menu option for E2 / Android is present with it
- Also E2 OpenATV v6.2 will be working via the boot menu after this install
Does not work with the 19-09-04 versions!
Known Issues List previous ATV8 test
1. CEC feature now working as with ATV7
2. AOL (Android Over Linux) available via online update
3. Backup/Restore feature available
4. The MediaCenter remains UNAVAILABLE (making it in effect still a Beta)
A huge problem for me is that none of the ATV8 versions support ISO images.
In fact my player is absolutely useless that way and had to go back as really all my stuff is in ISO format. 
Everything else seems to work (or works even better) than under ATV7.
The change from ATV7 to ATV8 in fact is one of the biggest in the ATV history. The main menu is now working with channels which are highly configurable to one's liking.
An upgrade is recommended only if one can live without the HiSillicon SDK provided Media Center as file manager and without ISO image support. It's a pity it does not work as it is far nicer than the simple alternative included (the same as with ATV7).
Running experiences will be added where and when applicable