One is the implied greed of the reader: „They [thank-you notes] improve the frequency and quality of the gifts you receive.” As if expressing gratitude was intended to yield gifts! Maybe for someone, but that’s far from my reasons to say thank you.
Also, the formula that is guaranteed to „never fail” makes a number of hidden assumptions, e.g., that the gift is meant to be used (point 3). What if I live far from sea and get a stone from the ocean from my friend who’s just returned from there? What I’d like to say is: „Dear John, thank you for the stone. I like to think that you had thought of me as you picked it up. When I look at it, I think of you and how important part of my life you are. Thanks for being it.”
There are better formulas. Ones that work for me revolve around non-violent communication: read up Marshall Rosenberg or „The Power of Gratitude” by Liv Larsson for details.