I'm happy with Parallels.
They just released Version 12.0.1 (41296). Don't know if this update could solve your issue.
I also wouldn't be satisfied with the answers you got from Parallels Support!
My tech profile:
MacBook Pro (Retina, 13-inch, Late 2013)
macOS Sierra 10.12 Beta (16A313a)
Parallels Version 12.0.1 (41296)
with Win 7, Win 8.1 and Win 10.
I have no experiences running another Mac OS thru parallels.
I've generally been happy with Parallels as well. I'm running 12.0.1 also and it didn't help. It was just a very frustrating experience to have it not work as it did in version 11 and then needing to disable USB 3 compatibility to make a USB 3 device work.
Having their tech people doing a remote connection to my machine and seeing that checkbox for USB 3 staring me (and him) in the face but it was never touched.
Thankfully its a backup drive and performance isn't critical.