R2R DACs are dependent on the accuracy of the resistors. Therefore if a particular design has a long term problem with one or more resistor values changing (absolutely or relative to the other resistors), then the DAC will be degraded. I suspect that unless the degradation is gross, most people would not hear the difference with normal music at normal listening levels. So saying "my 20 year old R2R DAC sounds great" is probably not a valid test! However, the degradation would be measurable and that would be a valid test. If this was a common, significant problem, I would expect there to be a lot of published engineering articles demonstrating it, but I've not stumbled on anything yet.
Downside to R2R Ladder DACs?
A sales person I generally trust told me to steer clear of used R2R DACs, since their reliance on high precision resistors causes them to sound best when new, and degrade fairly quickly. It seems reasonable; have others had any experience with this?