Actually did a presentation on the legality of corporal punishment some years ago.
I honestly don't think it's that bad. I think of everything I experienced growing up, being hit was probably the least traumatic. I cried a bit, I realised I'd done something wrong and I didn't do it again. In my case I think it was effective. I think it's okay to use it as a bit of a warning like if your kid is playing with knives and you need something instant so they don't get hurt, a (not dramatically hard) smack could help. Of course, I think an explanation and talk with the child to explain why is necessary - trauma is so easy for a child to experience if they are repeatedly hit without understanding why. Also if it isn't effective in punishment, it shouldn't be done. For me I learnt from it so I'm honestly glad my parents did it, and from my survey of classmates for my presentation, not one person who was hit suffered trauma or thought it damaged them. But if your kid isn't responding to it, hitting them for no reason is not okay. As long as it is explained, doesn't cause long lasting harm, had a reason and a positive effect in the long run, it can be used, but definitely not as a first resort. Personally I got more scared when my parents yelled at me than when I got hit, but I think my parents actually used corporal punishment appropriately, so I wasn't hit excessively or really hard. I have friends whose parents threatened to burn them with hot knives if they ate too much, so on that spectrum, I'd rather have a smack on the bum for something I'd actually deserve..