Whenever I play as Bison in HDR or E.Honda in SSF4, I can only use them a couple of times. After that, my thumb gets sore and I gotta switch it up.
Are you answering your own question or asking why the d-pad hurts your thumbs? Yeah it is a major problem with charge characters, you eventually will learn that you don’t need to hold down back so hard and that will lessen the soreness in your thumb.
Methinks it takes practice and patience, neither of which I have. That’s why, as a Chun Li player, I broke down and made a stick