Are there really people out there who are running PP2018 on High Sierra and NOT having problems?
Yes, I am having zero issues with it under High Sierra on a 2010 Mac Pro, 16GB of RAM and an SSD.
If so, how?
Many people disagree with this, but I truly believe it's all about never installing over a previous OS. I simply won't do it. When I read about all of the issues people come here for help with, and unless it's a genuine bug all users see, I never see any of these reported problems. Ever.
I saw proof of that just by going from the High Sierra beta to the official release. I was using a test partition for the beta. Installing the official release over it didn't fix many issues. Apps crashed, a lot of weird issues in general. You would think the official release would fix all of that. Installing 10.13.0 on an erased partition instantly "fixed" all of these issues.
It's a pain (mostly just really time consuming), but when the next major OS is released, I create a new partition and install it there. Then as I have time, I install all of my apps from scratch. I don't migrate anything, other than some preference files so I don't have to tediously reset all of my app's options the way I like them.
This does two things. One, all software is as clean as it can be. No old, orphaned files. No possibly corrupt apps or components (unlikely, anyway). If something doesn't work right away after this, then you know there's an actual problem with the new OS, or something a third party vendor needs to fix so their app works as expected.
The other, and bigger thing this does is it gives you a known, good setup you can boot back to while you're installing things under the new OS, and test that new OS thoroughly before moving to it as your day-to-day OS.
In short, always leave yourself an easy way back out.