Nah, in Korra many well trained fire bender could lightning bend. They even have lightning bender stations at power plants to add extra juice to the cities power grids. The key to lightning bending was kept secret for only the highest in society. Once Zuko and Aang come to power they teach it pretty openly. It’s not super common but not a super rare thing anymore. Same with mental bending. The rarest are still things like blood bending because they made it illegal and lava bending. Breath bending is the rarest though. One guy could do it in over 100 years and he was a psychopath anarchist that would have humanity living in huts fighting over each other berries because that’s the extent of anarchists ability to create a society.