Anong po problema sa coax? Salamat! 
Yung orig S/PDIF design (based on AES/EBU) kasi recommends galvanic isolation via a pulse transformer on both source and receiving sides. Unfortunately, sa kaka-costcutting ng makers sinimulang tanggalin ang isolation sa mga source, then eventually pati sa mga receiving devices. Instead of having a tranny isolate (and often step down) the output from the DAC chip, madalas resistors na lang ang gamit (voltage divider style) at madalas wala din circuit to block DC. It simplifies assembly and reduces the parts cost.
Mababa lang design signalling level ng SPDIF (0.2V min input as per spec... with many devices designed to accept even 0.1V), which is the same as AES/EBU. Madalas sensitive sa mga ground loops and stray currents ang receiving end, since ang design spec niya is to accept low level signals only. This is the reason why galvanic isolation and a DC blocking circuit would be beneficial (at one side at the very minimum). Sa pro components, EBU requires inputs and outputs to be transformer-coupled, pero sa mass market sphere eh caveat emptor.
In all fairness to most manufacturers, they are building wider tolerance inputs naman on mass market units to tolerate the possibly dirtier outputs of their cost-cut sources. Unfortunately, iba iba lang talaga ng tolerance ang mga gamit sa ganyan (hindi regulated, since beyond spec na pinaguusapan), and the end user might end up with the wrong combination.
