Thank you for your response!
So I guess I need to look further to see what problems will come up.
some 2015 users can't even get it to load, I wonder why I can.
One thing is my new iMac is all SSD .(256 Gigs)
reading around, I note that one of the distinguishing factors of High Sierra is
"When you install macOS High Sierra on the Mac volume of a solid-state drive (SSD) or other all-flash storage device, that volume is automatically converted to APFS. Fusion Drives, traditional hard disk drives (HDDs), and non-Mac volumes aren't converted. "
I don't really understand how the new APFS works, but I wonder if the fact that my iMac is all SSD
has something to do with my success with 2015 on High Sierra?
continuing to test.