He spent the whole summer getting himself in top condition for this season and still ended up injured doing a simple move on the pitch in not too hopefull of him staying fit for crucial games but I am aware that he is possibly our best player bar Ronaldo when on form.
Some of Costa's technical qualities are world-class as are some of his physical qualities like agility and pace. When on form he's right there with the best wingers in the world. His problems are that he hasn't been able to showcase those qualities on consistent enough basis and his end product isn't quite there with the best of them.
Berna too has some good physical qualities, but really seems to struggle with his decision making at the moment. I feel there is a player there that could score a decent amount of goals, but it just isn't happening for him at the moment. Berna is weird player in the sense that no one quite seems to know what his best position is.
I'm not sure I agree with your assessment on their potential. Costa's best form is better than what Berna has been able to showcase thus far in his career imo.