That's great that it worked for your aggressive dog, as using aversive training can make an aggressive dog much worse! Think about it - a scary man comes by and my owner pokes me/alpha rolls me, oh man, I'd better keep those bad men away so that my owner doesn't do that to me anymore! --> worse aggression.
I'm not going to explain all the reasons that have been posted before, I suggest you search "Cesar" on DF and read some of the posts for a more extensive list of reasons from more experienced and better trainers than myself. But just a few things - 1. Cesar admits this himself - he is not a behaviorist. He advises that no one repeat the methods that they see on his show without consulting a professional 2. Aversive training is old school and has since been replaced with more progressive and more effective positive methods. Aversives just make good TV. 3. A lot of the dogs he "fixes" by "distracting" them and alpha rolling them are actually just so shut down that they can't do anything anymore. It's not "submission," it's just him and the owner totally breaking the dog until it completely gives up and shuts down - not a healthy thing to do to a dog at all. Besides, the alpha roll has been disproven by wolf researchers anyway - the alpha wolf does not physically roll any other wolf, the others willingly assume the position because they respect the leader, not because they fear him. Being a bully, which is what alpha rolling and these "distraction" jabs are, does not make your dog respect you.
That's just a few things I can think of off the top of my head. It's a super touchy subject that has been discussed many times before (hence why you should search it) and many DF members are quite tired of it. Basically, the best trainers and behaviorists don't use his methods. If they work, you got lucky, because there is a much greater chance that using his aversive methods will make problems worse, and if used on a healthy dog, can create problems and totally break down the relationship between dog and owner.
Like I said, he does make some good points - like one I can think of is how he says he trains humans. But to me, the problems his training methods create outweigh any good that can come from the correct things he says. You can get those good things from positive trainers as well, so why not just get it from them and avoid all the bad/incorrect things he says? I'm sure he's a good person and he's doing what he thinks is best for the dogs, there are just much much much better methods, and I think it is very sad that he is so mainstream, as it causes people to think that he methods are the best when they definitely are not.
PS. I appreciate that you are coming into it with a general curiosity. I am definitely not interested in getting into a Cesar debate, but have no problem answering questions to the best of my ability and directing to threads that probably have better and more extensive answers than myself.
Also, I would really suggest reading Patricia McConnel's book "The Other End of the Leash" as it is a wonderful introduction to dog behavior, and a super easy and fun read. I wish that her book was as popular as Cesar Milan; if it was, I bet there would be a lot more happy dogs in the world.