Pistol squats, barbell back squats, shrimp squats, box jumps, I can name 100s of exercises better than Bosu Ball squats.

And well my legs might not be "massive" but I can squat 315 lbs at a bodyweight of 150 lbs - and perform 15 pistol squats holding a 15 kg dumbbell.

If Bosu ball squats are working for you, then good. For most, it's a bad choice