Your best bet would be to log it as a bug on the official bug reporting page (check the docs for the location, I don't recall it). The dev doesn't have access to all controllers, so it's possible the d-pad is sending out a different signal than Hat Switch. Then again, I've had problems with the d-pads on Logitech gamepads not hitting true direction easily and have adapted to using the control stick for games.
The reality is that computer joypads are just rubbish for some reason. I use a Playstation dual analogue controller via a USB adapter; it's a million times better than any computer USB pad I've laid hands on.
Also what kind of Mac are you using? Do you have any extra USB input devices attached, i.e., multiple mice/keyboards? If so it might be worthwhile disconnecting those. There have been interference issues in the past.