BOSU ball curls are idiotic. Why? Well...
- You simply can't lift as much weight when you're on an unstable surface.
- You lose a lot of the mind-muscle connection when your attention and CNS resources have to be divided into two different motor tasks.
As such, you're losing both the capacity to overload the biceps and to improve the mind-muscle connection when you add the BOSU ball to the equation.
"Yeah, but you're improving stability and core strength!"
Maybe. But there are better ways of doing that. And those methods don't involve making another exercise (curls) less effective.
But let's play pretend for a moment. Let's pretend that BOSU ball curls are effective at building the biceps while improving core strength and stability. They would still represent a progression from more simple balance and stability work.
After all, adding a second and completely unrelated motor task to a balance exercise makes it a lot harder to maintain balance. First because you're dividing your attention and CNS resources, and second because the curling action changes the combined center of mass which forces you to reactively contract some muscles to force your body to stay stable.
Curling on a BOSU ball is more challenging for balance than squatting (without weights) on that ball, which is more challenging than just standing on the ball. Yet most people who curl on the BOSU ball can't even stand for 45 seconds on the ball without having some movement. And they all look like they're surfing if they try to squat on it.
So even if it were a good exercise (which it isn't) it would be at the end of a balance progression. Yet most people who do BOSU curls have lousy balance. They look like a Parkinson patient on the Titanic! What good is that? If you can't do the exercise while being almost perfectly stable you're simply not ready for it.
In the end, you have an exercise that makes the curling portion largely ineffective at building the biceps, and a balance drill for which you're not ready for. Great combo, huh?