I'll start with the wholly negative effects of hitting children, specifically the section on Effects on Behaviour and Development. Time and time again, scientific studies prove there is literally no benefit to hitting children, with only poor outcomes.

My understanding is the most effective means of punishment involve first establishing an environment of rich support and love for the child. Then when there's poor behaviour, short time outs.

You remember that episode of The Simpsons when Bart steals the game cartridge, and what upsets him most is Marge's total loss of attention?

A secondary strong punishment is removal of positives, like revoking video game access etc.

It's hard to critique whatever parents you mention without knowing specifics, but it often comes down to:

• Poor follow through, with parents threatening punishment but rarely enacting
• Limited positive attention given to the child, likely due to "no time"
• Poor communication of reward/punishment system, or poorly established system.

Finally, sometimes children and just little shits with bad temperament. It's vital to understand that countless studies show physical abuse does not result in corrective behaviour, with only negative developmental outcomes.