I would be interesting to conduct the speed test on the exact same computer and hardware, but with Windows. That would verify Mac OS as the problem, which I think is very likely but not certain.
Installed Win7 on a spare SSD I placed in the spare optical drive bay.
Transfer speeds are all around 300-320MB/s due to Win7 not supporting UASP transfers. BOT transfer mode, even with registry "turbo" mods is still horribly inefficient.
With all 3 of the above listed USB 3.1 10gbps devices plugged into the USB 3.0 hub, everything is 100% stable.
Connected directly to the card:
With both the bus-powered Startech 2.5” SATA-600 to USB 3.1 10gbps enclosure and the Startech SATA-600 to USB 3.1 10gbps adapter cable, they're unstable, same behavior as under Sierra. Different outcome with the Startech SDOCKU313 SATA-600 to USB 3.1 10gbps drive dock; it's fully stable.
Now it gets weird: The bus-powered enclosure and adapter cable are unstable still under Sierra.
The self-powered drive dock is not only stable under Sierra, but it shows as Up to 10gb/sec. I had not yet tested in under Sierra 10.12.2 until just now. Transfers are running at 500-515MB/s. 511MB/s is likely beyond the real-world limit of a 5gb USB link. Screenshot is of Activity Monitor while copying a few dozen 2-6GB files from the Crucial MX300 1050GB in the drive dock to a Samsung SM951 512GB.
The bus-powered devices are still unstable at 10gb/s, however I'm fairly certain that it's a matter of not quite enough power available on the USB bus once 10gb/s mode kicks in and both the SSD and the JMS578 SATA-USB are both running at max power/speed.
Another thing I'm unable to isolate comparing the unstable bus-powered devices to the self-powered dock is cables. The Dock is USB-B, the enclosure is Micro-USB, and the SATA-USB cable is molded together. Using 3 different micro-USB cables, the enclosure is still unstable.
In general, I think our collective USB 3.1 10gb instabilities are due to a lot of cables that are just barely ok at USB 3.0, but fall out of spec when pushed to USB 3.1 speeds. Bear in mind that, even at 5gb, USB 3.1 uses 128b/132b encoding vs USB 3.0's 8b/10b, which will make USB 3.1 much less tolerant of supply voltage sag and lousy cables.
Minor update: The dock crashes after 250-300GB of sustained 500MB/s transfers. I pulled it apart. The ASM1351 bridge chip inside it gets up to about 105°C then it locks up. Added a small copper heatsink to the ASM1351. 550GB transferred without errors.
Ableconn PUSB31P2A USB 3.1 Gen 2 10 Gbps 2-port Type-A PCIe x4 card works and is currently $30 on Amazon.