I wouldn't include iMacs as professional desktops. If they gave you the option of a screen less one that was just the processing hardware so you could use your own screen, that would be different.I don’t fit the mold of a Pro user by many people on here, so I may not know, but are current MacBook Pro’s or iMac Pro’s not capable machines? Are they not able to meet demands of Pro users? Not talking ports or user upgradeable parts, since there are solutions to both.
However, iMacs are horribly designed hot boxes that cook their own screens (know from experience).
Not to mention horribly over priced for what you get in the package.
as for upgradability, there is none. Other than a little extra ram anyways. Buying external GPUs is not cost effective. Plus the aforementioned internal thermal issues and horrible air flow disqualify the iMac for me as a professional user.