IMHO there's a fine line between discipline and abuse.
Yes you may not know what goes on behind the four walls,
Yes I find it horrible when I have seen kids cringe at bring yelled or threatened with a smack.
But I still think a strong verbal tone - not swearing - then if not reacting a smack on the hand or bottom can be effective.
I also was 'smacked' as a kid, usually only when I'd done something actually naughty, most of the time my parents did the 1 - I'm asking, 2 in telling, 3 time out/smack depending on situation thing - it seemed to have worked, because I can remember doing the wrong thing very clearly and getting smacked for it.
I don't consider this abuse. I don't remember the pain but I clearly remember the why I got it.

Another angle of abuse is silent treatment, and ignoring the child being rude, naughty or violent also.
I know of a grown adult who works in a corporate environment but had to go for counseling to overcome how to deal with conflict, as he had never heard his parents yell at him, or gave him 'rules' when he was young. Granted he grew up a great person, not in any trouble with law/drugs/alcohol, in fact became a Dr (academic) but still baffled when people have a difference of opinion or argue, and still gets anxious.
I have seen kids hit kick/smack/make the parent bleed, for the parent to do nothing but embarrassingly give a wry smile and give an excuse that their child is tired/age or being cheeky. Teaching children abuse is wrong, is right, and if this means sometimes shocking them a tad with a stern No please do not do that, and a smack on the hand, it usually resonates.

This is not to cause debate, just a different opinion.