I think a lot of people just think that a few sessions of crossing and that's the problem fixed! Professional football players cross the ball at pace and accuracy. You don't realise how hard they hit the ball till you are close up. If Fox was good at cross balls he would not be playing for us.
That Dundee United goal was almost certainly pre-rehearsed at training before the match, Shankland stays at the back post then when the corner is kicked comes across Fox blocking him from coming out. The rest of United players stay out of the line of the ball isolating Shankland against Penrice.