I used virtual box for many many years and it was great until this year when the 2018 computers did not support properly and it’s a slow as molasses. A Minute between clicks for it to recognize was too long of a wait . Parallels is supporting the modern architecture’s of metal.
This must be specific to Mac. I've got a mid 2018 testing rig that now runs LM 19.1 and the new 6.0 VB release and it's actually even faster then it was before. Some of the features that were labeled as "unstable" for Windows guests are now "stable".
But if this is an issue with the Mac hosts, definitely go with Parallels if going with newer hardware.
I would suspect it's due to Apple's deprecating of OpenGL and forcing people to use Metal. A lot of open source projects are going to be hit by that that depend on OpenGL (Blender, Krita are the big ones for me), of which VirtualBox does use.
Why I didn't go with Mac after I left Windows as a host OS. Even if my Linux fork of choice removed it, I could put it back in (easily). Not so easy with the others.
We ran Flexi on it with no problem. I had bought a Dell specifically for the purpose of running Flexi but it ran it terribly and had a serious lag when receiving the size of the media from the Mutoh. Ended up going with virtual machines on the Mac and it moves along much faster.
Resources, resources, resources. I have had far better stability with Vista in a VM (even using Aero and the unstable VB 3D graphics driver) then I ever had on bare metal (physical hardware, not the Apple video api mentioned above).
It’s also great to be able to transport simply the machine from one computer to another. This is true no matter what the host system is.
One concern to keep in mind. The VM files are portable (depending on the program, it may not be cross platform though for the host unless convert it to the ova extension), however, when swapping from one computer to the next and that new hardware is "seen" by the guest that wasn't the original, it may "break" the OS install and/or the individual program installs as well.
Sometimes it happens, sometimes it doesn't, but just keep that in mind. Flexi may not be a concern, but it's something to be on the look out for the Windows guest OS.