First off, that disarm ruling is just...bad. A Will check AND an opposed roll? It really should be one or the other, not both. How does he justify not having the willpower to hold on to something but you are still physically strong enough to do so? Or the opposite - you have the will but not the strength?
And I agree with leons1701 - the power, due to your GM's rulings, has been rendered to being next to useless. I'd ask him, nicely and politely, why he is making this ruling on the power, and bring up the points leons1701 brought up above. He's the GM, and it's his game, but as a player you have the right to question why he is making the rulings. Things should be about balance, not about eliminating an option because he can.