He is probably talking about good old (synchronous) NRZ.
With NRZ, you can't recover clock unless you can guarantee that there will be some minimum transition density, i.e., if the logic level never changes for a long period of time, clock cannot be recovered. Analog Devices has **broken link removed**, which has a patented phase detector, but it only works between 20MBPS and 160MBPS, and it requires thousands of clock cycles to acquire lock.
EDIT:
After rereading the OP, I realized that he specified "embedded clock", so this is probably not NRZ.