If there's one problem with Bostic's tackle form, when you check the video you can see that at the last second he ducks his head towards the ground, so that the top of his helmet is facing the target. IMO, that's the problem if there is one. You have to keep your facemask up and pointed at the player if you're going to hit him hard, that way you're not using the crown of the helmet to enhance the impact. If your eyes are on the defender where you're hitting him the whole time, it should be a legal hit, and Bostics facemask lowered to the ground just before impact.