Other than the coolness factor do you really need to run more than 13 GPU's off of the same motherboard? I know there is economics of scale, but these motherboards cost more as well, offsetting the savings gained. Plus you need extra PSUs, more memory and a bigger SSD. So adding all this up the cost savings really isn't a lot more than say 2 smaller rigs.
Also, a point that has been argued before, if the rig experiences issues, then you have 19, 21, or however many GPUs you manage to get running down at once versus with smaller rigs you would have maybe 8 GPUs down while the others would remain running. While these videos are nice to watch for the geek factor, it seems to be pointless after a certain limit.
This. You have the redundancy and up time factor of having multiple rigs and not working every piece of hardware component to its fullest capacity all the time. Decent MBs, mem, and a PSU are the inexpensive parts of a rig anyway.