I read somewhere (sorry don't remember where) that canards controls for missiles are more effective than tail controls. Is this true, and if so, why?
Since the control unit sits at the tip of the missile, it is simply more practical to control it via canard deflections. The rear part can then be made as a simple pressure vessel with fixed fins and a nozzle.
If the rear fins would have to move, their actuators would either be in the way of the nozzle flow or would stick out on the outside - in both cases performance would suffer. Also, long cable runs along the side of the missile would need to be added.
By controlling the missile with the forward fins, any change of direction will be direct, whereas control via the rear fins means that the rear end of the missile would initially move away from the desired direction, until the change in attitude would move the missile towards its target. Thus, the canard will result in a more agile missile.