I finally got hold of an EXSYS EX-1188HMS (sorry, 1189 was a typo), which uses the Genesys Logic GL3520. The hubs mentioned by ilovezfs sound like good candidates as well, but don't appear to be available in Germany.
Currently I have been running diglloydtools DiskTester for over 24 hours on an HFS volume: Mac mini Server Late 2012, EX-1188HMS USB 3.0 hub, "Intenso Memory Box" drive, which uses a Seagate 3 TB ST3000DM001-9YN166 and JMicron USB 3.0 bridge (0x152d:0x0539).
DiskTester has been doing read/write/verify with various data patterns over the middle 1.7 TB of the drive and is reporting very consistent data rates of 160 MB/s write and 110 MB/s read. No errors, and the connection seems stable.
This looks promising. Next I'll try connecting a mixed bag of USB 3.0 drives, building a ZFS volume, and hammering that for several days.
Here's how the hub reports in System Profiler.
USB3.0 Hub:
Product ID: 0x0612
Vendor ID: 0x05e3 (Genesys Logic, Inc.)
Version: 90.14
Speed: Up to 5 Gb/sec
Manufacturer: GenesysLogic
Location ID: 0x14b00000 / 4
Current Available (mA): 900
Current Required (mA): 36