12 GPU hashcat rig
Has anyone seen this?  It has been created for the mining community.  It seems to be that it could also serve the hashcat community.

12 GPU motherboard

What is the current conventional wisdom regarding the use of risers these days with hashcat?  I seem to remember reading in these forums a couple years ago that using risers with GPUs for use with hashcat wasn't optimal or recommended.

(Nevermind that the GPU market is in a shambles right now.)
The best GPU for hashcracking is the 1080Ti. You can build a machine with 8 of these GPUs with a server chassis and PSUs for ~12K. It does not make much sense to save a few bucks with an amateur mobo that brings you into trouble in cause of multiple PSUs and a complicated setup housing 12 GPUs.