There is one inherrent problem with all card based clocks AND converters.
It basically comes back to a big, and very underrated issue, which is the quality of the power supply.
Cards do NOT provide sufficient high quality power for a good power source, it is simply impossible to integrate a stable voltage / supply into a computer card.
Therefore it is logistically impossible to build a card based A/D, D/A, or clock, on a card.
Take this one step further and ask yourself if a quality, well balanced power supply for a converter can / should be the same as that for a clock. Once again, the answer is no. Different animal.
Therefore the only realistic solution is to use an external source of power. Either built, well isolated, into an external piece of equipment, or, as for instance is the case with the small Lucid GEN 6 clocks, use a power source remote from the unit itself.
Another example of the importance of power supply, as well as the analogue circuitry design of a converter is that a lot of the 'higher end' converters like for instance Apogee, Lucid as well as others use exactly the same chip. Yet there is a huge difference in sound between Apogee and Lucid. The difference is in the units power supplies and analogue circuits.
The above are all reasons why I always advice to first of all look at clocks and converters if you think of system improvements. Believe me, it is more important than pre's, mic or anything else. Use a Shure 57 through a well clocked system with good converters and it will sound a million times better then for instance a 6000 dollar mic with a 2000 pre recorded through a system lacking accurate clocking or converters.
My first clock, bought a long time ago, cost $8000, a fortune at the time. Now you can buy something like a GEN6, a far, far superior clock than my first one, for around $500. For that 500 bucks you get a much better sounding system - period.
Last, a lot of people have in the past commented "I tried it and didn't hear 500 bucks worth of difference". Let me try to explain why you might not. By using a good clock source your material WILL sound better going to disk, period. However, how much of the difference you hearm immediately depends on the quality of your converters and monitoring system. The better your system, the more you will hear. If, over here in the studio, I turn the master clock of, so the system defaults to the generic clock of whatever piece of gear would take the lead, most likely a Pro Tools
HD192 I/O, you'd immediately think "what the fuck happened to the sound?", as you would hear the definition disappear, the high-end going to pot, and previously defined low end notes turning to a mute rumble.
However - if you can hear detail or not, getting it to disk in the highest possible quality will give you better material to work with - period, which is after all the end result everyone is after, regardless of the cost / quality of their system?
I hope that helps to explain the reasons why