I'll take a guess that the bridge chip in some of these adapters aren't so friendly to non-x86 implementations of IDE, which would be all the pre-PCI Macs equipped with IDE. There is zero reason for needing faster cards when working with old machines. They are all going to be bottlenecked by a slow interface, most of which are PIO Mode 0 (its doubtful Apple went any higher on 630 series machines) and top out at 3.3MB/sec. Almost all of the speedup comes from the near-zero seek times that flash media has compared to hard drives.