Considering the author of the JDM admitted this back when it first appeared, it's obviously the fault of the design - and he never pretended it wasn't.
The JDM was purely a (very clever) design to use the absolute minimum of components - reliability and repeatability were never really concerns. It also relied on the serial port actually exceeding the RS232 spec, which many old machines did.
It's unfortunate really, the low reliability of the JDM has probably put more people off PIC's than anything else?