Apple maintain security updates for at least 2 previous revisions of macOS. As long as you keep your system updated you’re as secure as the current version.
There are also numerous incidents mentioned by Google Peoject Zero, if you read through their macOS disclosures.
It’s definitely unsafe from a security standpoint to run anything other than the latest release.
Still, running something like little snitch, behind a router, along with some browser plugins can go a long way. Until Sierra, I have never had an upgrade that went smoothly, something critical I used has always broken.
Unfortunately Apple's OS updates are as bad as a zer0 day exploit in how they can knock one's setup inoperable, sometimes for many weeks or months until there's a way to fix, or revert.
Software vendors I'm using seemed to be out ahead of the High Sierra train which was encouraging.
You do backup things right? Disk cloning through Carbon Copy Cloner is super fast and restores are painless.
Apples backups were the reason I switched to Mac from windows a decade ago. Can't be beat.
I run Time Machine on one hd at office, carbon copy cloner on another hd at home, all they the USB hub.
It's not a backup if it doesn't exist in more than 1 place.
I was referring to the case where half your apps don't work and the other half benefit greatly from the new version.