As good as these external USB audio interfaces are - I recommend them almost universally, not any particular brand, but just to get the analog signal path entirely outside of the noisy computer box and keep it in something that's actually designed intentionally to do what it does, and do it well - there are still a couple of annoyances, mostly to do with signal routing.
Why in the world, for example, would anyone want *the physical interface* to be a loopback to the computer, when every computer OS already does that? Look for that setting, and always have it off or disabled. That function *there* can only hurt, from what I've seen. If I were making one, I'd remove it entirely.
Another gotcha is how the interface's headphone out is driven. I *can* see a use for that setting, in a few niche situations, where you want the headphone out to either ignore the computer's signal and only pass through the interface's local inputs, or mix the local inputs with the computer's signal. Most of the time though, you want it to only have the computer's signal and *not* the local inputs at all. So look for that setting too, and make sure it's right. If you do want to hear the local inputs through your headphones, see if it works first, to have the computer loop them back, without the interface itself doing it. Then you hear what the computer actually gets, which is usually more useful...if the latency is low enough.
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For a computer's built-in headphone / line-out, that's usually a last resort for any serious work, if even that. It's an analog signal inside of a noisy box, so it doesn't matter if it has stupidly high-spec chips in it. It's still going to have about 10 bits or so of useful dynamic range, if even that. That's still enough for rough spot-checking, for only a few seconds at a time and then you put the headphones down again, but that's about the limit of its usefulness for serious production work.
And I *have* seen the built-in audio device fail. So if you can't make it work, then that's a possibility too. Depending on how it failed, the symptoms could be almost anything, since it has both digital and analog components, and the software might have no way to know about some analog problem.
