I have to rest it on a pillow if I go for too long while playing in handheld mode. We've actually been using it alot more docked to our TV now, so it's really not even an issue in that setup, it's only when you're holding the thing itself. Going back and picking up a Switch feels surreal now though, almost like it's too light or it would break easily.

I've found the same. Using the Switch in bed with awful posture my hands would cramp after about half an hour. Using the Deck in bed with the same awful posture my arms fall asleep after an hour but I'm still going strong.