I'm very on the fence with this whole issue. I haven't smacked either of my children in a long long time and I am the first to admit that there are other methods of discipline that work. They require a bucket load of patience and require a parent to be very very stubborn and not give in, they require calm, they require clarity and a strategy and consistency. My children are not angels by any means however, since I starting working on my own strategy, looking at outcomes, what I wanted from a punishment, how to achieve it etc., our house has been a much calmer one, and my own headspace has been a lot clearer.
I will not write off smacking completely though. When my eldest was younnger, he was becoming quite 'fisty' with other children, when he wanted something he took it, and after months of gently gently discipline I smacked him hard enough to sting on his backside. He never EVER hit a child again, he realised that everyone has a line you don't cross, and he also understood with few words WHY hitting isn't nice.
I felt shit of course, I felt a failure. I felt guilty and dirty. From my experience, with very rare smacking, it does not become a case of monkey see, monkey do. Though I accept that if smacking is one of few forms of communication, that would be the case.
I think where any discipline become unfair, is when its not accompanied by discussion, explanation, and reason. That goes for the naughty step, smacking, withdrawal of treats, confiscation... clarity is key.
Same with shouting, shouty parenting is not where I'd like to go, but every so often, a shout IS appropriate, effective and right.
My children know when I am angry, when they have taken too many steps over the line, and I do think that's a good thing. They have to know that if you keep pushing the button, its going to have consequences. They have to know that if you keep putting your hand in a croc's mouth, one day its going to bite.