Magnetic locks seem to be fairly common on the doors of modern buildings, but I question their reliability when compared to a traditional, mechanical lock. I realize that the strength that it would take to actually pry a door away from a good electromagnetic lock would certainly be enough to damage the door or frame, just as it would be with a mechanical lock.
If you had an electromagnet strong enough, however, wouldn't it be possible to negate a magnetic lock by fastening your oppositely-polarized electromagnet on the outside of the door opposite the magnet on the inside so that, when activated, the outer magnet would overpower the attraction between the inner magnet and the magnetic material on the door, effectively opening the locked door?
If so, I feel this would be a pretty big disadvantage when compared to a traditional mechanical lock. Not that a powerful and portable electromagnet is necessarily a readily available and affordable thing, but...
Ignoring the practicality of actually owning and operating such a powerful electromagnet, is this even a plausible thing to do?