Coaxial Digital Input vs. Optical Digital Input  

As a very broad generalization:
Coax links could be susceptible to airborne interference. The most critical parameter for them is a consistent 75ohms. While this is not hard to do in the cable, often time the RCA connectors are off.
Optical links don't suffer the above but their potential problem is the optics themselves: In the same way that a camera lens can be cheep and yield a poor picture, so too can the optics at the cable ends and in the components themselves be compromised.