I think I understand the warning, after he revived the Queen his ants stopped obeying him aka loyalty doesn't change the inherent nature of what you summon. Same with the healer dude who healed the girl without being commanded to. Same with Beru he was winding up for the killing blow even though he was told not to. If what he summons has a strong enough Willpower then the summon being doesn't have to do what Jin says. If he tried to summon this crazy dragon for example, who is an intelligent being who only wants for the end of the world like Alduin then he would accidentally create a threat to humanity.