It is all about the connector impedance. RCA plugs are very difficult to make at 75 ohms, a 75 ohm BNC is a snap. Nagys cable make a digital cable with a proprietary RCA connector that is almost exactly 75 ohms. (While this helps with the male end I would think you are still at the mercy of your hardware manufacturer as to the impedance of the females jacks on your equipment which is why BNC is still a more reliable solution) This may account for some of the very positive reviews it has gotten, in spite of it's sub $100 retail price. Rollo had a demo piece recently and absolutely loved it.
Here is a technical explanation from the Nagys web site:
RCA Impedance
This is a phenomena that happens only at higher frequencies.
In consumer products the load is 75 ohm and in military it's 50 ohm.
So, when you have a cable that goes from your transport to DAC,
everything must be 75 ohm, this includes the female RCA connectors,
male RCA connectors and the coax cable itself.
If one of these components is not true 75 ohm part of the signal will
reflect, but only at high frequencies, like the ones digital signals are
transmitted in.
If the cable it 75 ohm and the connector is 70 ohm only a small
fraction will reflect, however if the cable is 75 ohm and the RCA is
40 ohm (which is typical for most expensive/fancy so called audiophile
RCA connectors out there) significant part of the signal will reflect.
First, this reflection makes the signal weaker and second, the
reflection can cause an avalanche effect where it disturbs the signals
behind it.
This is all very bad for an instrumentation type cable.
Most RCA connectors are 40 ohm. At high frequencies, the
shape of the connector and the thickness of dielectric will determine
the impedance. That is why it's so hard to make RCA connectors 75
ohm, their shape and conductor spacing prevent this. BNC connectors
are a different story, they can easily be made to be 75 ohm.
The full text can be found at:
http://www.nagysaudio.com/digitalcabletech.html