sounds cards still far superior to onboard sound although onboard has improved a lot over the years.
but your dont get the same high quality components used on the boards as the cards. There is good reason Asus's top sound card costs £175. You dont find they fit £175 of sound circuitry to their £200 boards
Although that board you linked to comes pretty close, as good as it gets, but I presume that is their top of the range board???
Often routing for the traces is a bit of an after thought with onboard sound and you end up with longer than ideal traces and/or bad layout with regard to transmission lines, etc. also what saddens me with onboard audio is you'll see like a decent audio processor but then coupled up with a ropey DAC or they use a nice DAC but a barrel scraping c-media processor, general purpose capacitors in the signal path and/or cheap and nasty op amps, buffers or drivers.
Many of the better external DACs are hard to beat these days as they are often built around the PCM179x which does very well for consumer grade and you really need a specialised high end part to beat.