Some kids are distraught when separated from the family, others are quite content. IME timeout can be far more emotionally damaging than smacking. A well timed smack is a punishment for action; a "timeout" appears to be a desire for separation originating with the little-person's most loved-ones. A short-cut to the brain telling it that a behaviour is unwanted (smack) vs. an emotionally loaded action telling them [seemingly] their loved ones don't want them around.

I've smacked and ended self-destructive behaviour immediately and permanently (with ongoing reinforcement); done timeout and simply made a child distraught and supremely insecure.

Horses for courses, as they say. [That is, people vary and what works for one person will not work for another.]

Slight aside, isn't all correction for "harmful things" just at different levels of harm? I don't for example correct my children's "poor manners" in putting their elbows on the table because I don't consider it harmful in any substantial way; presumably my parents found it harmful enough to chastise me over otherwise why would they do so. My presumption is that they felt I'd be somewhat excluded by polite society for failing to adopt societal norms of table manners.