That works when they are teenagers with the consequential "it's not faaaaaaaaaiiiiiirrrrr!" Kevin & Perry routine that follows. Unfortunately you can't reason with a 3-5 year old and that's the point the short sharp smack works after fair warning. Boundaries are then set.
My 2 are typical teenage girls. The elder can be controlling of her younger sister and youngest reacts by being passive-aggressive which takes a fair bit of nipping in the bud which I leave to Domestic 0A because that's easier and the better dynamic when it comes to teenage girls. What we try and be is consistent, and that's the key. If each parent tries to behave differently or plays their differences of opinion out in front of the kids then that's when the trouble starts.
My niece on the other hand.... my in-laws are amazed that whenever she is with us she is no drama, but at home she is demon-child. That's largely due to my SIL being a wokey push-over who has all kinds of faddish diet ideas that constantly change, all kinds of faddish parenting notions and BIL half the time confused about what the rules are today to even try and keep up so he just says yes all the time. As he's away a lot with his job niece gets subjected to gluten-free, non-sugar, no dairy free-range slop she won't eat which results in tantrums, melt-downs, and then being presented with other non-dairy sugar-free options, so she's learned to keep kicking off until she gets what she wants (which always ends up with ham and eggs, but has to be presented in a certain way or she won't eat it). BIL comes home, dinner on table, niece kicks off, gets chips and pizza to keep her quiet, parents row, niece sits on her phone/switch/iPad. Try and take those off her, get her to go to bed on time, and again, tantrums that turn to slappy-kicky-scratch fests and they try and "gently reason with her as a burgeoning adult". She comes to us, she eats what she is given or goes hungry and goes to bed when told. 30 seconds of confusion as the "why won't Aunty Woopert just give in" before the penny drops and dinner is eaten, rewarded with a few treats, then bed. Thankfully she is an only child because my SIL has a sister who was (and still is) clearly favoured by her parents and that leads to a whole other story of f*cked-up.
The fact is parents make a rod for their own backs by just not being consistent and trying to follow the advice of some Yank quack who wrote a book ignoring the inconvenient truth of how their own sporn turned into self-indulgent train wrecks because of the faddish parenting they promote in their parenting books.