Using a medicine ball for squat depth is acceptable for Wall Ball, high rep air squats and learning progressions. We don't prefer to use them for Thrusters, especially with intermediate and advanced athletes. Targets are never used on Back, Front and Overhead Squats.
Everyone should strive to develop a distinct feel for what depth is appropriate (safe) for them. For some, 14" will be too high while for others, it may be too low. At the end of the day, we all know squatting to med balls is easier than not, so use them sparingly.