I think the question of smacking should be looked at in a wider way than just as a physical thing, because what can be said against it can also be said against verbal violence - shouting and screaming at children. What is wrong with both ways is that they can have traumatic effects. They come under the heading of "fast solutions", and those are nearly always damaging in any area of life. For example, in gym training everyone should know that it's a bad idea to do too much too soon. People who impose too big a load on their bodies in order to achieve faster progress always pay a price in terms of disturbed metabolism, pulled muscles, damaged joints, and so on. To progress more slowly gives the body time to absorb the training and development is always smoother.
I think it's the same with children's minds. They are very easily traumatised. To try to correct their behaviour with some kind of blast of emotion - either by smacking or shouting at them - is trying to achieve change instantly. You might succeed in the short term, when the child ceases (for the moment) to misbehave, but there could be an emotional price somewhere, even if you can't see it immediately. There will be some kind of hurt, some kind of resentment, or fear, or other negative emotion. And by accumulation it could be personality-changing. They could become more obstreperous than they were before, or they could go in the opposite direction and become withdrawn. You can't predict it.
Obviously, some people seem to survive a childhood where there was lots of smacking without apparent damage (but you can only take their word for it). But when dealing with children you can't make calculations like that. Even if only one in ten was psychologically damaged, that should be seen as unacceptable. And you'd certainly think so if your child turned out to be one of them, wouldn't you?
It isn't just the smacking that's wrong. The impatience is wrong. It's saying to a child "Why aren't you perfect already?" Why should they be? Their minds are still in the process of formation, and trying to force the pace is bound to produce negative side-effects. Dealing with children is the most difficult job in the world, and there is no substitute for patience. You just have to keep on gently correcting them, while setting a good example yourself. They'll get the message eventually.