I've start building my new rig with the following configs:
Asus Prime Z270-P
PNY 1070 ti
Inno3D 1070 ti
PCIex1 to x16 ver008s (with multiple power options)
Win 10 Pro
Mobo Bios set to:
enable 4g decoding
Gen 1 for all PCIe
disable RC6 (Render Standby)
disable unused SATA (don't think this matter on this issue)

The issue is that Inno3D card won't be detected (not showing up in Device Manager) when setting up via riser. I've tried on the same driver with PNY card and it works alright. I wonder is there any compatibility with riser and specific brand GPUs. I've try forcing the bios to boot through the GPU by setting the bios to GPU->PCIE and plugged my Inno3D to the x16_1 slot through risers, and nothing happened (black screen, no flash); but with the same setup of PNY GPU on riser, it boot up fine and loaded the screen. The Inno3d cards can function perfectly when plugging them straight into the x16 slots.

I've tried using DDU to clean the drivers, and disable all windows update. And also trying to plug things in different slots and sequence. Noting seems to solve the incompatibility.

Currently I've be setting my two Inno3D 1070 ti straight on the two x16 slots and the PNY through riser. Everything run perfectly and I can alter them to get each 30+ MH/s.  But I still I wish to have set them all through risers (still go a few more Inno3D cards).
I also saw a few other saying that a certain card can't be detected (ASUS OC....)  Please any suggestion will be appreciated.

Any update here?  I got some 1070 ti cards that also aren't detected via risers, but they work on x16 slots on board. I've checked the risers and they work with other brands.

I gotta ASUS Z270-P with 1070 ti. One specific brand (Inno3D) isn’t being detected on risers, the other brand (PNY) is working well. The non-detecting ones work fine straight on x16 slots. Will be trying Linux based miner to give it a go.