I've ran into several issues, but I'm not sure any will help you.
1. m.2 share lanes with sata connectors on some (all?) boards so if you are running a hard drive it can interfere.
2. Running a dual PSU to them is finicky, better to run gpu, riser, m.2, and mobo off same psu.
3. You can split the m.2 4xpci for AMD but not Nvidia. I've ran 6 AMD cards off 2 m.2.
1.I am not sure about that. m.2 is using x4 pci-e lanes from "south bridge".
example: South Bridge: Intel Sunrise Point Z170
PCI-E 2.0 x4 port #9 In Use @ x4 (Samsung NVMe SSD Controller)