I can only comment on my experiences from the USA to the UK - I have brought back 3 guitars on this route, all with United Airlines - all came back in cabin, in the cupboard where the flight crew keep their stuff. Packagin technique was guitar in hard case as usual, layer of bubble wrap on the outside of the case. Top tip - don't cover the handle, then you can carry it....
No problems whatsoever on any of them.
I've seen loads of people take gig-bags as carry-on luggage but not hard cases.