Nadal is amazing on hard courts when he's at his top level, but when he plays average, he's much more vulnerable than on the others surface. Djokovic is not better on grass, probably worse despite his Wimbledon title.
Federer on clay = Nadal on hard (the field is way tougher on hard courts than on clay) > Djokovic on grass > Murray on clay
hard courts are the most relevant surface in tennis, its no surprise that players don't put anywhere near as much effort into grass and clay
wimbledon right now is without doubt the most competitive-less
grand slam, hence why thats where murray is most likely to vulture another slam, NOT in AO and USO where he'll get real competition like delpotro and djokovic