Common Knowledge
You will delight your customers by underpromising and overdelivering, i.e. going above and beyond what the customers expect.
Rebuttal
Test: underdeliver, deliver to the letter, overdeliver on promises.
Overdelivering produces little meaningful increase in gratitude or appreciation.
A promise is a contract; you expect what was promised to you and nothing more (otherwise you would’ve asked for more).
Delivering to the letter is essential for maintaining reputation [MK: which is more important for the word of mouth than delight].
Generosity is not as important as fairness.
[MK: generosity advertised via the word of mouth leads to unrealistic expectations by new and existing customers.]
There is another point (not mentioned here) which is applicable in professional services, 'Underpromise and Overdeliver' is not a good approach when you have "super" stars in your team who can overactive the results, but then when a team member needs to be replaced and another one may not keep the bar of that individual.
And what could happen even worse, that overdeliver becomes a new standard.
Therefore, it's much better to master the predictable outcome without any margin on both sides.