It is often considered an advanced kata, despite its brevity. Karate master Shoshin Nagamine considered wankan to be his favorite kata. A quote from his book describes the kata as "Being characterized by unitary sequences of attack and defense".
Redmond, R. Kata: The Folk Dances of Shotokan, 2006 (http://www.24fightingchickens.com/kata/)
Nagamine Shoshin: The Esscence of Okinawan Karate-Do (Paperback)