I think it largely depends on the person. If I rest my hands naturally on a traditional keyboard, my fingers end up somewhere around wdfv okjn. To pull them around to the home row basically involves tilting my hands outward and bringing my wrists inward even further, neither of which is good for either your hands or your typing speed.

If I have pain or numbness (either of which can last for days), just having my hands rotated to use a flat keyboard for a few minutes could be difficult.

Generally, though, if I get pain in my left hand/arm, it's from excessive typing; in my right, excessive back-and-forth to the pointing device (I use a trackball at home if I do any gaming, pad at work, just so it's not the same device all the time). I have never really been able to use a mouse.