Standing or moving on a balance board or a Bosu ball isn’t going to improve ankle strength unless you’re in the early stages of ankle rehab after surgery.
Building strength requires significant resistance. Doing strength exercises on a balance board will limit the amount of weight you can move, as most of your focus is on staying upright.
To really build ankle strength, focus on single-leg strength exercises such as single-leg squats and single-leg deadlifts. Even step-ups.
Yes, you might spend some time in the beginning to get your balance right.
But once you get a hang of it, you’ll get way more out of it than trying to balance on an unstable surface.