BS and Sin are not challenging and hard to play, what are you talking about? The Assassin outputs rapid dps even at low pvp level and the BS is a tank with a ton of resources at hand to keep himself alive, especially as AA and gear unlock. The ToS is probably the hardest of the 3 and only because everyone knows a ToS has to run in to a center of melee to DPS where at the same time, he gets destroyed for doing so, so everyone stomps on the ToS class and kills him. The class essentially isn't hard to operate or play, it's just broken within game design due to everyone knowing how to kill one in group play.
There is nothing challenging about AoC, especially with AA, Gear gaps and the current gen of combat mechanics.