Mechy wrote a reply recently that was rather thorough and I have 100% confidence in his products and ability to sqaure things away if/when there are problems. I don't believe SCSI to be "black magic", but I have run across my fair share of completely idiotic and unexplainable behaviors dealing with SCSI all these years.
Latest one has to do with a Plextor CD writer where the 2091 card stubbornly refuses to boot the HD quickly as it would if there were a non-writer installed. With a CD in the drive, the 2091 card *does* allow for a quicker boot, but is still not as fast to get off its ass upon a reset/cold boot. And yes, all settings and config scenarios have been covered. *This* is the sole reason I'm even considering one of his SCSI<>card readers, BUT... I've got another question about his reader, which I'll do in a PM. 
The best and only way to get great scsi is to have it on the accelerator, its usually night and day speed wise. All these old cards like the 2091 do work,but are zorro2 and slow. Adding at least 512K ram or more to the 2091 makes it quicker also,since it can DMA to it.Updating the 2091 WD scsi chip helps when you are using more than one drive and impliments the full scsi II command set properly-Problems can creed up with more than 1 drive with the old WD-04 chips(proto makes no diff). There were many buggy amiga scsi cards out there and some accelerators with bad scsi(apollo cough!)
the zorro3 A4091(rev 7 roms) and fastlane z3 have good scsi though.
Most problems can be easily explained.
Mech