I use a Wacom tablet, and have one huge PITA that I've never been able to solve. If you use the size jitter assigned to pen pressure, the brush size gets bigger and smaller. There's no feathering setting, instead PS uses "hardness". The problem is that hardness is relative, so if you set the hardness to anything other than 100% you get different amounts of feathering depending on the size of the brush.
This is a problem most often when I'm masking and painting the mask channel. If I have the brush's hardness set to give me the perfect edge softness/feather to match the focus on an object, but then ease up on my brush to get in a smaller spot, the brush gets harder and doesn't match the rest.
Verrrry frustrating. I've been using Photoshop since v.3 and Wacom since their introduction and this has always driven me nuts.