Ahhhh, here is the major problem with dog training. Although all the methods work for some dogs unless you understand the way dogs learn and the language they speak to each other how can you say what is cruel?

The question runs far deeper than most are prepared to go. If it feels O.K. to a human and the animal shows no recognisable signs of distress then, for most people, that is enough. Even if the distress is recognised but is short lived it is often deemed a necessary process by many animal trainers. (Take a look at horses and how they are broken in.)
I do not advocate any of the training methods I mentioned but I believe that small parts of some of them are useful in certain situations, for certain dogs.

Dog training is not rocket science - but it does depend on the owner finding things out for themselves. Nobody will know your dog better than you.....BUT it is soooo important to know that above all it is a wolf and thinks like a wolf and understands as a wolf. Corrections have a place in dog training but it is important to know that physical punishment is a last resort in a pack and frequent physical punishment is only ever seen in unstable packs or those with poor leaders.
It is a subject for each to make their own mind up on, but the further into the questions you go the more questions you find.