I use Adaptec 2960U to connect two scanners and two 50-pin SCSI HDs along with an internal SCSI HD (used for backups daily) with no trouble.
I don't trust any USB-SCSI and it can't come close to 5-10MB/sec speed.
One thing I did have to do (even on my old PowerMac) was to use smart SCSI active terminators. One for safe mode to switch scanners on/off rather than have to reboot or leave them on all the time. Another, APS SCSI Sentry II, to add power, monitor SCSI chain, add active termination. Also allows longer length than the 1.5 meter length and better signal with less loss or noise.
Cost was $99 plus cost of high density 50-pin SCSI cable to connect to the Adaptec card.
Greg