A few years ago I came across a guy at LAX airport ( Los Angeles) with a nicely padded gig bag. He looked familiar but ,like an idiot, when talking to him I failed to ask his name.
Anyway, this guy flies all over the place gigging. He takes......a beautiful, clean, near mint '57 Les Paul with him!!!!! He says he has never had a problem taking it as carry on luggage. Usually the flight attendants let him hang in in the suit closet, or they put it in an overhead and then steer others away from the bin. He has never been told he had to check it for any reason.
I have a hard shell case for all of my guitars and only one, the Strat, is probably worth close to $1000 replacement cost.
Gave me the shivers seeing this thing in a bag.
But he says he's never had a problem.
My preference has always been hard cases, but there are some well made gig bags out there nowadays.
Just my opinion....YMMV