Granted to someone who is not familiar with Ra's and his history and connection with Bruce would make that connection, that he is a weak villain with a silly concept, but the fact of the matter is that Ra's is a global threat because of his global aims. He's targeted Gotham because it is Bruce Wayne's hometown and understands the symbolic importance it has to Bruce. The League of Shadows has operations all over the world. I think Liam Neeson made him work because Liam understood the character's motivations and desires. It might not seem on the outset that Ra's is Bruce's greatest enemy because of the long drawn out conflict between Bruce and Joker but Ra's is a more personal threat, even moreso now since they're family.