1ns or more is pretty typical for normal devices. EMU0404 has like 7.9ns on eyepattern, my dacs have no problem locking on to that. But yeah 1ns is pretty darn terribad for a device build for the purpose.

As for why, there could be many reasons, impedance matching, connectors, implementation, all that goes beyond just using low jitter clocks. Like the Geek Out devices have a fair amount of low frequency jitter despite using special clocks.