From a functional user perspective, the electrical contamination/noise is the biggest con, IMO, which regularly surfaces on onboard implementations for analog out. I build a few computers every year, and the issue still occasionally surfaces. In my experience, it's either there and is incredibly noticeable, or I don't really hear it. It even comes up with discrete sound cards. I had the issue with a Xonar DG card in a Corsair D800 case, where I routed the DG to the front-panel via the Gigabyte HD Audio connection, and the static was very loud and prominent. Meanwhile, it was to my ears, non-existent from the back of the card.
My primary rig's signal chain is a WaveLink HS into a Berkeley Alpha DAC (as well as experimenting with various ways of getting the signal into the Alpha before I got a WaveLink), so I'd say I'm regularly exposed to what the high and low-end of computer audio has to offer. No, I haven't tried the SOtM tX-USB yet, but that and rubidium clocks are beyond my point of caring.
I think source is important, but for the needs of 95%+ of the population (and that includes HF), I really wouldn't recommend something that much beyond the quality of a gamma2 or equivalent device. In my experience, diminishing returns kick in quickly, and as far as headphones go, it takes a Stax rig for me to appreciate the difference. I've heard it's more evident on a top-tier speaker rig with proper room acoustics, but I'm not there yet.
I occupy a minority position where I shake my head at reading $5k+ source reviews that come off like a bad romance novel, but also am not happy with most sub-$500 retail offerings I've come across, and that includes the sound cards and opamp rolling regularly cited as amazing. If I was making a recommendation to someone, I'd tell them to get a good enough $500-1000 DAC that punches above its weight class, and spend the rest of their budget in amps, tubes, and even a preamp as necessary. Even then, I think, as long as noise isn't an issue, onboard audio or a cheap sound card is more than sufficient for the majority of people. What a lot of folks consider fidelity is not really fidelity.