I added the RebTech 8GPU-MB:
Very interesting product and company. RebTech seems to have existed since 2015 (
http://rebtech.ro) but they are now designing their own motherboard and even a custom GPU and they are advertising them on a new website (domain name
http://rebtech.ai bougth 2 months ago.)
The mobo is very compact and has 8 PCIe slots exposed as USB ports (no need for risers on the mobo side). And RebTech also made a custom GPU with a USB port instead of the PCIe edge connector. So combined together, the mobo + custom GPUs (RX470 8GB) offers a completely riser-free setup.