There are definitely hazards but done correctly even very extreme ballbusting can be pretty safe. Use the top of the foot rather than the toes to distribute the force, don't mess with the "strings" and you're usually fine. You have a lot of nerves there so it's painful, but not necessarily damaging.

I have given many a hard ballbusting in my day and I have seen a lot of clients who take hard ballbustings routinely. Overall ballbusting-related medical issues are almost shockingly uncommon.

I've done extreme ballbusting shoots and gotten angry tweets from guys accusing me of faking it or being horribly irresponsible. Nah, it wasn't fake and the guy was fine. He's just a tank.

Notably, both trampling and breathplay have considerably higher risks and get a lot less hand-wringing.