Just thought I'd feedback and share my experience;
XG125 running MR8
Upgraded to 17RC, got a warning it would take some time, fair enough!
also got pop up saying you could view status in log viewer, very cool!
not sure what exactly happened but after an hour I ssh to the box as it wasn't web consolable or passing traffic.
tried to reboot without success.
Had to physically reboot the firewall.
everything started functioning as usual and then i got a notice saying firmware available.
somehow the unit reverted back to mr6 with mr5 as its backup image, when i loaded RC it took the place of mr7 (i think) leaving mr8 as the live image and just leaving RC to click boot which i did to initiate the upgrade.
I have no clue why!