We can't be sure that we will always be 100% correct, and we don't want to "criticize by writing" to cause complaints, so the best way is to express our different views euphemistically.
First of all, customers will be unhappy when they hear rejection. Whether it is reasonable or not, sometimes they can speak in another way.
Second, speech has a purpose and results in communication.
Let others know what the focus of your speech is in the first sentence, then clarify your ideas one by one at the first, second and third points, and finally make a summary.
Third, "Shut up" is sometimes a way of communication.
In the workplace, the most talkative person does not mean that he is the most capable person. In the wrong time, in the wrong place, in front of the wrong person, sometimes silence is worth a thousand words.
Especially when you meet unreasonable customers, don't argue with him. You should know that the argument is endless, and it is impossible to have a normal conversation with a person who is obsessed with the dead end. You can let him vent, and when he wants to ask your advice, you can give a reasonable explanation.