What is the toughest sport to master?
I’d go with auto racing. Not that I’ve ever even tried, or that I’m even a fan. Never tried, and not a fan.
But you’re driving a death-dealing machine at high speeds in a close pack of others doing the same. One mistake by you or by the other drivers and you can be severely injured or severely dead. Not a whole lot of room there for an error.
For my second choice, I’d go with cliff diving. High probability of injury? Check. High probability of death? Check. Not much room for error? Check. But at least you don’t have to depend on others not screwing up.
Now, I realize that the question did not ask about the most dangerous sport. But it’s tough to master a sport in which a simple mistake while learning how to master it can render you unable to master it because you’re dead