User blog comment:Susan Ae/Is TAME lame?/@comment-1600847-20130429154408

It's said a few time in the books that dragons can only be controlled by sorcery or those Dragon Horns the Valyrians used. It's shown in A Dance with Dragons that Daenerys can't control Drogon, i.e he'll only obey her if he feels like obeying, if not then tough luck. Her dragons won't harm her, but neither will they obey her, making them a very questionable and dangerous weapon.

Imagine her using her dragons in a battle only for them to attack both her army and her enemies.