Are old SCSI tape drives not all created equal?

Hi, folks. I'm experimenting with various old SCSI tape drives to see which will work with my PDP-11/34 with an Emulex SCSI card. To my surprise, not all SCSI tape drives are created equal. I was under the mistaken assumption that all SCSI tape drives would pretty much be abstracted the same way by the SCSI interface. Drives that are working fine are: - Archive Python DAT - Exabyte 8mm - Cipher 995 9 track - any of the DEC branded DLT drives Drives that aren't recognized are: - Teac MT-2ST (this drive seems completely nonstandard -- can't even use it as an ASPI drive under MSDOS) - OnStream Advanced Digital Recording (ADR) SC-30 (doesn't work under MSDOS / ASPI either, but does work under Windows 98(!) with a special driver. Won't work under WinXP with same driver. - Quantum branded DLT drives (work fine under MSDOS / ASPI) Question: So, even though some tape drives physically have a SCSI interface, are they different in some other way such as to require special software to use them? Or maybe there were different SCSI standards? Or is the standard simply imperfect? Any commentary, appreciated. Thank you. - John