I have run across a few ESVs at my field that were off significantly. So far I have not personally seen any digitals (like the MKIII) that were off.

The ESVs have internal potentiometers, which can be adjusted for better accuracy. The digitals have no adjustments, just one or two ICs inside. I have found the digital meters, even the most inexpensive ones, to be spot on. Personally, I don't care for the ESVs.