I have two road bikes with Di2 and one with Campy Super Record EPS. My problem with my hands is/was carpal tunnel, not arthritis. I put a thread in Road Cycling a while back complaining a bit about shifting difficulties with Di2 because of my carpal tunnel. There are several different shift switches available with Di2 which offer alternate placement on the bars - someone suggested I try those - I had carpal tunnel surgery instead in August on one wrist and the other scheduled in 3 weeks. It was easier for non-mechanical me than dealing with switching shifters.

So - while I complained it still wasn't easy to shift Di2 with completely numb hands -- it was still somewhat easier than shifting some of my other bikes with various Shimano or SRAM mechanical group sets.