My understanding is that the 2013 's GPUs are a similar GCN architecture to the AMD Sapphire 7950 Mac Edition so actually they’ll run games just fine, they just aren’t as fast as newer GPU designs. They aren’t CAD-specialised GPUs (if such things even exist anymore) - they are just custom designs/daughter-cards to fit inside the more limited space and thermal/power envelope of the new .
Note that UE4 can only render with the primary display GPU in the new because OS X doesn’t have CrossFire.