My first idea is that it’s probably outside the dimensions for carry-on luggage. Not that this stops a lot of people.
Though a unicycle can get crushed, it’s no more susceptible to these crushing forces than the rest of your luggage. Keep the cranks on (pedals off) and they protect the sides of the wheel, as well as keeping the hub from poking through your luggage. I usually surround mine with foam and other stuff in the bag that helps pad them.
As Unicorn said, we don’t want to every lie to anybody. Airline personnel just don’t need to hear the word “cycle” when checking in your stuff. Never, ever lie to customs people (they don’t care unless you’re breaking import/export laws), or security people.