You loose speed in ATA, in SCSI it’s less of an issue because of the agressive use of the disconnect feature which gives each drive nearly simultaneous access. Finding “completely compatible devices” is not a feat – SCSI works with SCSI, it’s as simple as that. And as far as legacy versus “newer” devices, most non-disk devices run over a narrow (8-bit) bus (or the low byte), disk interfaces use a 16-bit bus with a high byte to double performance. Also, with U2W and above, the bus can operate at different speeds simultaneously, allowing for no issues with legacy devices … that’s right, it can work at 5 MHz Synchronous SCSI (SCSI-I) all the way up to 40 MHz Synchronous SCSI-III (U2W), and Ultra 320 controllers expand that limit up to 160 MHz Synchronous SCSI. Furthermore, you can connect more devices and internal AND external devices.
Onto termination, it’s used in ATA/66 and higher, you just don’t hear about it. Indeed, newer SCSI devices will automatically terminate the bus if they detect that they’re at the end of it. In add to the ease of use, the SCAM protocal means you don’t have to set jumpers for the ID (or eve Master/Slave like in ATA) at all, the device can be hot-plugged onto an existing SCSI bus and automatically configured without even the OS interveneing (the controller arbitrates the bus itself).
To add to performance benefits, Ultra 320 controllers sucking down full-speed data from 4 15,000 RPM drives uses under 5% of a 1 GHz PIII … doing the same thing with 4, slower, 7200 RPM ATA drives utilizes over 50% of the CPU, which means you need to buy a beefier CPU to see the same performance. And we won’t even get into how much faster or reliable the SCSI disks are …
And on price, yes, you pay twice as much, but in a $5000 to $10,000 box the disk subsystem will likely only be 10-20% of the cost of your box, and you get a choice: pay less for ATA but that means you need to pay more elsewhere for the same performance, or pay more for SCSI and get the same price/performance ratio and have 4x the reliability of an ATA subsystem.