Suspend to ram should be safer than suspend to HDD. Mostly because if a newer kernel is installed, only the old kernel will accept the hibernation image.
I think powerfail during a correctly-executed upgrade is expected to be recoverable. I don't know exactly what level of resources you need to guarantee that. Basically, you want to be able to boot, log in, and re-run the same apt command e.g.
(This is not true of rpm. rpm is expected to not handle powerfail gracefully; it requires manual intervention).
I think as Bruno9779 points out, the main reason not to rely on this is that you should be expecting something to break anyway, unless you've already upgraded a sufficiently similar system. If you're not already experienced in recovering from the powerfail, you could be dealing with two new problems at the same time. Stressful, and with the possibility that they compound each other.
The upgrade is not 100% guaranteed safe, unless you're already prepared to give up and do a fresh install. As the ever-helpful upgrade instructions say
Although Debian tries to ensure that your system stays bootable at all times, there is always a chance that you may experience problems rebooting your system after the upgrade. Known potential issues are documented in this and the next chapters of these Release Notes.
For this reason it makes sense to ensure that you will be able to recover if your system should fail to reboot or, for remotely managed systems, fail to bring up networking.
and the documentation for the recovery system it points to conclude
Repairing broken systems can be difficult, and this manual does not attempt to go into all the things that might have gone wrong or how to fix them. If you have problems, consult an expert.