I believe that it's very difficult to talk about one-size-fits-all approaches to parenting. A study that finds that spanking doesn't improve behavioral outcomes on average is going to be a study that is aggregating a lot of different children.
However, the difficulty with any violence, even very gentle and limited violence, is that it always teaches children that the way to deal with people they don't like is to hit them. Even when a parent can hold back their anger so as not to teach a child that it is ever a good idea to hit someone else in anger, it's still very difficult to do so successfully.
I was spanked once. I did something wrong but I was also a child and what I did was based on my inability to communicate my feelings at an adult level. Punishing someone for something that they don't have the capability to avoid is already a difficult pill to swallow. Doing so with physical violence is even worse.