... Micheal De La Masa ... was one of the few low rated player to go to expert level studying tactics only; ...
http://www.chess.com/article/view/the-michael-de-la-maza-story
Apparently, he had studied openings. Also, it is perhaps worthwhile to consider how long he played at the expert level.
The player who understand opening principles and he is a beast in tactics and endgame can play any opening with proficiency because he can calculate better and his evaluation is more accurate than his opponent who only concentrates on opening.
Can anyone expect to quickly satisfy the yureesystem standard for beastliness in tactics and the endgame? What happens when facing an opponent who doesn't "only concentrate on opening"?