alll risers support 3.0 (x1). well if motherboard support only 2.0, than the riser will do 2.0 as well.
Its simple the usb3 cables dont have the quality to negotiate pcie 3.0, think about it usb3 cables are made for 8b/10b same as what pcie 2.0 run, but pcie 3.0 use 128b/130b.
Now there is faster standards like USB 3.1 Gen2 or USB 3.2 that do use 128b/130b but i would imagine that they also have much shorter cables, and its not cheap
EDIT: Already at pcie 2.0 these usb risers suck, if you have problems with gpu fallen of the bus errors in your syslog, and your gpu's are not too much clocked, you can usualy fix it by forcing pcie 1.0, so the risers are iffy even for 2.0, you say they can 3.0 I say NO !
8/10 vs 128/130 has absolutely nothing to do with this. That is simply the encoding for DC Balancing/ensure there is a periodic bitflip on the transcievers so they can stay locked. It was changed in PCIe 3.0 due to improved transceivers and to lower overhead.
Also for everyone checking lspci - most GPUs now adays are designed for low-power idle. They run the link at x1 unless they are actually being used for transfer! Run a bandwidth heavy test or activity and run lspci, youll see LnkStatus kick up to 3.0 (8 Gbps)
We just spent weeks testing dozens of risers and slots on 3 dozen motherboards. I havent found a single riser that cant sustain PCIe 3.0 speeds. While monitoring PCIe for any errors. I have, however, found crap mining motherboards that have PCIe errors on their slots even with a x1 device plugged straight in. Ill take that actual testing over your anecdote.
Software (miner) and APIs: This is a Pre-Order for the hardware. The software will launch with the hardware shipping late August. Weve already asked devs to reach out for API and SDK information that will be available before launch. Developer boards havent even been sent out yet to other devs.