As the 1st Assistant Editor I am frequently importing and transcoding hundreds of gigabytes on my 2015 iMac 27. We can easily shoot 500 gigabytes per weekend across six cameras. It is fairly quiet, but I have multiple Thunderbolt disk arrays on my desk so that tends to swamp any iMac fan noise. It is much quieter than my 2015 MacBook Pro, which spins up the fans at the slightest touch.
Re alleged negative reliability of hard-core editing, this is frequently mentioned on some forums. However that is mostly supposition and speculation by people with no engineering background. A feature film recently had a bunch of New Mac Pros fail due to supposed thermal effects during protracted rendering. They were all replaced or repaired under warranty.
If those nMPs were defective or being pushed too hard, then after repair they'd fail repeatedly when put under the same stress -- they didn't. It was probably a manufacturing defect isolated to a certain production batch -- something like improperly applied thermal paste, etc.
The same thing applies to the iMac. It should not fail or have hugely shortened operational life from normal or even heavy duty production use.
Re what specific temperature, I have iStat Menus but I mostly don't look at the temps.
That said, there are definite advantages to the nMP. It is quieter, and has much more available CPU and GPU performance than even a top-spec iMac 27. The design is three years old so hopefully in the relatively near future there will be an updated Mac Pro.