Chance Miller, 9to5Mac:

With WWDC every year comes new beta versions of all of Apple’s operating systems. This year, we got iOS 12, macOS 10.14 Mojave, watchOS 5, and tvOS 12. All are currently available to developers and are meant for testing purposes. After all, they are betas and are buggy.

With beta releases come a handful of issues. Devices typically run warmer than expected and suffer from shortened battery life, as Apple typically doesn’t really optimize performance until later betas.

And:

Furthermore, you’ll likely run into app compatibility issues across all of the latest betas. The purpose of the beta period, again, is for developers to test their applications and services on the latest versions of macOS, iOS, watchOS, and tvOS. This means apps aren’t optimized and might not run properly on the beta.

That last is the biggest bugaboo for me. I can live with occasional crashes or quirky behavior, and the current slate of betas seem exceptionally stable. But for certain apps, compatibility is critical. If I move to a beta and a key app just doesn’t work, I’ve wasted a lot of time since I’d be forced to restore from a backup to get back to where I was before I took the leap.

If you are considering a backup, check out this Mojave compatibility Reddit thread. Be sure to read the comments, lots more issues revealed there.

If you depend on a 3rd party device, like a specialized trackball with programmable buttons, say, do a bit of research to see if the device still works in the beta.

I am more likely to take the leap with my phone, since that does seem stable enough with all my key apps. With Mojave, I’ve still got a bit more research to do, since I use so many specialized apps.

Obviously, make sure you have solid, reliable backups. For your iPhone, make sure you do an archived backup, which will ensure that your backup will not be overwritten by a backup of the beta, just in case you have to restore your phone back to 11.4.

And finally, keep in mind that embracing a beta is a choice. If you make it, don’t complain about things not working right. That’s the world you are entering. Stuff will break. But it will (in the vast majority of cases) get fixed, and performance will no doubt get much better as debug builds are replaced by performance builds. Report the bugs you encounter so the folks doing the heavy lifting can follow up.