Especially if they have older 32bit applications and a mechanical drive. Yeah, if it's a personal user device and everything is going smooth, I wouldn't rock the boat by pushing the upgrade to Mojave now.
I won't typically start testing until they're at a. It doesn't really pay to be an early adopter. They're not always completely unjustified. In general, major Mac OS revision updates pretty much always get horrible reviews.
Mojave seemed more resource intensive and changed the security model a bit which made some older applications incompatible.
