I mean, you can Google it and find countless sources, if you really care they are readily available within seconds.
The Tl;dr is that his methods are based in dominance theory. Dominance theory has been widely debunked and the methods that arose from it are widely considered to exacerbate fear and aggression related issues in dogs. Caesar’s celebrity status has contributed to its persistence in the popular imagination.
That’s really a modern thing. It used to be that you’d buy a nice PC and 3-4 years later it can’t play new games at an acceptable frame rate and resolution.