The concern isn't with upgrading both or not upgrading both, the concern is with one of them (most likely the phone) being upgraded, and the other one (the Mac) not being upgraded. The phone will try to upgrade the database. Is that optional? Or will it just do it?
The phone will upgrade the data on iCloud, the Mac will not be able to use the new data on iCloud, and the two clients will no longer reflect the same data because it's CloudKit doing the syncing.
This was why I was trying so hard to keep tasks in IMAP, vs keeping anything in iCloud. It's unfortunate that's been abandoned (hint), so much of what makes GoodTask useful to me is repeat after completion and auto-snooze (especially, allowing me to replace both Alarmed and Due!), and those are gone unless you use iCloud (having to maintain them multiple places means you just don't use them). If you're keeping score, one tic mark from me in the "who cares if there is JSON getting inserted in the notes field" column.
Now, I have no interest so far in upgrading the phone to iOS 13. The claimed increase in performance on the same hardware looks interesting, but the rest of the junk... meh. However, it is extremely difficult to stop an iOS upgrade from getting incredibly naggy. So I guess I'll have to wait and see.