@Log @jtredux Right. Physically compatible connections with different pinout should really be illegal somehow… 
I was rather hoping for a way to connect generic PCIe x4 or x8 devices via these cables. I was considering making my main EPYC rig micro-ATX, but then it seems I’m short on one USB controller.
So the ideal solution should be a USB controller directly connectable by ASRock’s “Slimline SAS” cable, or second, an x4 or x8 PCIe riser connectable by the same cable (to which I can then connect a USB card). PCIe gen3 or even gen2 should suffice, so signal integrity is perhaps less of an issue.
For now, it seems my best bet is something like (Slimline SAS->U.2 cable)->(U.2->M.2 enclosure)->(M.2-connected USB controller).