This is a slightly facetious post because I really don't think it is that simple or that there is a definitive right or wrong answer...
I shout at my children sometimes. Not a lot or excessively but, for example, if their behaviour is very poor I am likely to shout at them to stop it or tell them to leave the room or similar. We're not talking shrill screaming, but we're certainly not talking calm, "please stop doing that now" either. I have noticed similar with my siblings (not surprising I guess) and also a few friends.
I'm starting to notice that people who appear never to shout at their children, also seem to have a lot of behaviour issues. I suspect the real truth is that SOME parents who don't shout, ALSO don't have enough boundaries of any form and that's what's really happening, but from the outside, it just seems like the super patient kind ones are also the ones with the most badly behaved children.
In one case, I am actually concerned about a friend and her DH and their relationship. Every time we see them, her DH appears more and more worn down and exhausted and he has said a few times it's because he just so overwhelmed from the children and their behaviour. HER response is to say that the children are behaving that way because of x or y (usually sounds totally realistic so I understand where she's coming from) and that it's not their fault - and the dynamic is very much that he is NOT allowed to discipline or shout at the DC. The problem is that these are pre-teens so when the 9 year old is stealing food, making secret online purchases, refusing to allow anyone to watch TV if it's not what SHE wants to watch etc... even if it IS because she's exhausted and overwhelmed, its an issue. And her sister is similar but different - lots of screaming and shouting and refusal to listen, hitting out when frustrated etc.
This is an extreme example and a family I'm genuinely worried about as the mum is a very good friend, but I also saw it a lot over Christmas while we were out and about, "Rupert, please stop banging that lady's leg" accompanied by a wan smile at me as Rupert continued to whack me and so on.
In the case of my friend, there have been instances where I've been with them and my DD has behaved similarly, and I immediately jump on the behaviour. So DH and I are considering whether we need to spend less time with them simply because DD is not getting boundaries enforced there either.