Incompatibility between SCSI host adapters and SCSI devices is still a monumental problem. I speak from experience. Many manufacturers who produce allegedly SCSI-2 devices actually produce incompatible SCSI-2 devices.
Worse, SCSI hard disk prices are 2 or 3 times the price is same-size IDE hard disks.
No matter how many devices are actually connected, only one pair of devices can communicate with each other at any one time.
Device 7 has the highest priority, so the host computer is usually assigned to be device 7.
The SCSI lets all devices communicate with each other, but some devices are implemented in such a way that the cannot initiate communications.
Single-ended is limited to a maximum cable length of 6m.
Differential supports cables up to 25 m in length. Differential is much more expensive to design and manufacture.
In the original SCSI specification, synchronous communication allowed speeds of up to 5 Mb/sec. Note that if your SCSI bus is short, asychronous communication can be fast.
SCSI-2 supports 8-bit, 16-bit and 32-bit buses; also it supports up to 10 Mb/sec on 8-bit buses, and thus up to 40 Mb/sec on 32-bit buses.
5 Mb/sec 10 Mb/sec 8-bit Fast 16-bit Wide Fast-Wide 32-bit Wide Fast-Wide
The SCSI-2 specification allows fast SCSI only on differential SCSI buses, but in practice vendors sell single-ended fast SCSI devices.
Also, it will run at up to 20 Mb/sec.
SCSI-3 will include a specification for serial SCSI over fibre-optic or high-speed copper cabling. Speeds aimed for are 51 Mb/sec up to 1 Gb/sec. These serial proposals are called IEEE P1394 (nicknamed FireWire), Fibre Channel and Serial Storage Architecture (SSA).
Another advantage of serial is that fewer wires are required, simplifying cabling. Frankly though, mixing serial SCSI and parallel SCSI looks like a recipe for disaster.