Millan's techniques are almost exclusively based on two techniques: Flooding and positive punishment. In flooding, an animal is exposed to a fear (or aggression) evoking stimulus and prevented from leaving the situation, until it stops reacting. To take a human example: arachnophobia would be treated by locking a person into a closet, releasing hundreds of spiders into that closet, and keeping the door shut until the person stops reacting. The person might be cured by that, but also might be severely disturbed and would have gone through an excessive amount of stress. Flooding has therefore always been considered a risky and cruel method of treatment.
Most of the theoretical explanations that Millan gives regarding causes of the behavior problems are wrong. Not one of these dogs had any issue with dominance. Not one of these dogs wanted to control their owners. What he was right about was that calmness and consistency are extremely important, but they don't make the presented methods appropriate or justifiable.