You can get strong using box squats. I've seen people only do box squats from the very beginning and still make good gains. If you are progressively overloading the musculature, you will adapt and get stronger. But the question is, are box squats the best way to get strong. For me they were always more of an assistance exercise, and they have their place in my training. But I always have to keep doing some form of low bar squatting too, especially since I compete that way.