Downside to R2R Ladder DACs?

Really! I have two R2R DACs that either are 20 years old or uses DAC chips that are from the era of highly praised chips from Burr Brown and  Phillips that sound "pristine" with no change in their performance at all. So, your sales person is giving you a bunch of BS, in my opinion. Question, does he only sell Sigma Delta single bit DACs, if that's so that might explain is unfounded assumption regrading R2R based DACs.

30 year audio professional really high quality resiseors wont drift you are mutch more likely to hav a problem witha cap or ic

Assuming the R2R DACs were built with genuine fine tolerance resistors this is utter nonsense.