Hibernate is effectively power-off as in same battery drain as true off. IIRC, it puts a flag in bios (or the bootloader?) to go straight into hibernation recovery where it loads the on disk copy of ram back into ram. Basically skipping all the initialization and first time loading stuff.
Fast boot is some janky "we'll just hibernate the core services and everything else is off" junk that just causes more problems. I felt a difference back in Win10+HDD but I don't anymore with SSDs. They really should get rid of it as it always causes unnecessary troubleshooting mysteries or at least turn it on conditionally.
As IT, I don't enable hibernate because its not as fast as sleep. I don't want to deal with the users who complain about the startup speed. (I'll have to investigate again as the modern laptops might be fast enough that it's ok again) I do however allow sleep but it's also so crap that I now get "the laptop is asleep but when i came back the battery is dead!" ugh. win some lose some i guess.
Yes but also dont let perfection block progress.