An iMac screen is just a perfectly ordinary consumer-grade IPS panel, just like any other consumer-grade IPS monitor. The Apple logo doesn't make it anything special.
That it doesn't come anywhere near reference-grade monitors like NEC PA or Eizo CG/CX should really be no surprise.
Except is a freeking 5K monitor driven by a single controller. The Apple logo doesn't make it anything less special either.
But I agree that is not for color critical workflow. Now in the other hand you can buy an iMac and a external calibration monitor.
For very heavy use, in the other hand the Mac pro is better starting by thermal considerations. Also in terms of grow potencial.
Now, I will like to see Apple upgrade the Mac Pro before investing on it but that my "evaluate the WCC" engineering mentality.
Jokes aside, the iMac is two generations ahead of the Mac pro in terms of processor architecture /node combination.
The other advantage of the Mac Pro is how easy is to upgrade, but the next generation Xeon from intel will not fit the current mac pro.
Best regards,
J. Duncan