If you follow the gaming notebook forums much at all you'll see that the 6-core 8th gen chips are very difficult to cool properly even in much thicker frames than the MacBook Pro offers. Seriously, the Notebookreview forums are full of people upset that monstrous gaming notebooks aren't able to cool these chips properly, especially if there's a discrete GPU also pumping heat into the system.
As soon as I saw that these Macs had been released with these new CPUs I started wondering what kind of cooling magic Apple might have worked to make it possible. The answer seems to be: none. The laws of physics are the laws of physics and if these chips run hot in 1"+ thick systems with massive heatsinks and fans, what chance do they have in something as thin as a MacBook Pro?
The thing is these MacBooks were designed for quad core i7s not hexacore i9s. I think this situation is similar to what happened with the 2011 MacBook Pros that had the Core 2 Quads and discrete GPUs. They had originally been designed for the Core 2 Duos. I think the extra heat from the CPUs was what caused the Radeons to keep failing in those.