Because the disarm displays an icon and can't be dispelled, you can use it to reveal your enemy before it uses a Manta Style or ability to create illusions. It can be very useful against Phantom Lancer.

Can halberd be dispelled by BKB?

Once the halberd has been cast on a hero, they cant be dispelled by BKB or Manta which is not balanced.

Does halberd pierce spell immunity?

Does not pierce Spell Immunity. Dispellable.