The reason is because the cheap OEM motherboards they use do not always come with PCI-E certification.
People who attempted to run an RX 470 in a computer like the Acer you mentioned ran into power issues - the system would simply not boot with the GPU. It lead them to speculate that the motherboard's PCI-E slot cannot fully deliver the specified PCI-E power probably because it might not be validated against PCI-E-SIG's standards.Can you explain what that means?
P.S. One desktop I am thinking about is the one discussed in this thread (Specifically the new Kabylake version).
People who attempted to run an RX 470 in a computer like the Acer you mentioned ran into power issues - the system would simply not boot with the GPU. It lead them to speculate that the motherboard's PCI-E slot cannot fully deliver the specified PCI-E power probably because it might not be validated against PCI-E-SIG's standards.
The sources are Wendell@Level1Techs and the Tech Deals guy.