If one takes a step back to look at the disarming separately, I thought that the example in the rule book (YS208, where a maneuver is made to place an Aspect, DISARMED, on an opponent) was a bit strange and seemed too powerful at first. But after thinking about it, the following two things made me both understand and accept it (without the need to spend Fate Points to actually disarm the opponent).
First, see the Aspects as “hard facts” – this is some kind of “truth” about the game world (not just words or phrases that might be true). This is true for all Aspects, not just the ones created through maneuvers (i.e. being DISARMED is as true as having the High Concept Aspect WHITE COURT VAMPIRE ASSASSIN). This means that if the opponent has the Aspect DISARMED placed on him, he is disarmed (in addition to the fact that the Aspect can be tagged as usual). For him to remove the Aspect, he has to make a maneuver and get rid of the Aspect (i.e. take up his weapon).
The second thing is that the maneuver is done instead of an attack. The roll is opposed by the same (or at least similar) skill (not a fixed difficulty), meaning that the “attacker” is placing an Aspect on the target instead of causing stress on the target. At the end, the attacker gets a disarmed target and an Aspect that he can tag instead of a number of stress that could take him out of combat. In case the attacker would like to try to disarm an opponent using his bare hands, the difficulty would be increased even further.