I've noticed that in many cases fast reboot (reboot -f) but also reboot to BIOS (reboot -p) on Solaris 11.4 guests fails. The guest goes to "aborted" state. It's not much of a problem as powering on the guest boots the guest in (apparant) normal way, so it is not much of a nuissance.
In a few cases there is a message which may (or may not) be related:
For example when installing a guest from an Automated Install server from a UAR (Unified Archive) the above WARNING is printed, at the very end after "system will reboot now" (at the end of the succesful install).WARNING: Fast reboot is not supported on this platform since some BIOS routines are in RAM
This is not something that is broken recently, it is an issue that is already long like this (I think) but I thought to mention it again.
Also the fact that reboot -p also goes to 'aborted' state (for the guest) shows that the message of the OS is perhaps misleading and that's it a different issue not related to fast reboot, because the reboot to BIOS is similarly bringing the guest to 'aborted state'.