I have never had them weigh it and you can pay for a 'hand luggage' guarantee.

You can still have an issue as I did.

I flew in June with the guaranteed hand luggage - No. 5 in line. The lady checking BPs grabbed the bag and told me it was going to get checked as the bag did not have a label. I explained I paid for the guarantee - it shows on the reservation but not the boarding pass - and that you only get the label if checkin-in at the desk. Pretty non-sensical. I had medical stuff in the bag so calmly told her I had to take it on board and she threatened to call security! So I left the bag - boarded - asked the co-pilot for help - and he walked back with me to get my bag.

Badly trained and ill mannered.