I'll dive in since surprisingly nobody else has bitten. I rarely fly to the US.

But at TPE I've never seen it checked at check-in. I check-in at the gold counter, but just asked a friend who said they don't generally check at the normal check-ins either.

I HAVE seen them pull a few PAX at the gate where they've too much hand luggage and the flight is full. Pretty rare and they seem to target those that are obviously over: eg a large roll-on plus another large bag etc.

Also seen a similar thing happen at LHR and SGN. Was once told that their unofficial weight limit was 12Kg in the case of a single carry-on.