Dear NoEffects -- Welcome aBoard!
I tend to parables myself and so to illustrate use of underwhelm :
Mother is expecting a ticket for a luxury cruise to England aboard the QE II for her birthday, Instead, the adult children present her with a ticket on a local river cruise.
Mother is undewhelmed. (Of course she smiles and says thank you...never letting on. That's what Mothers do!)
It's an oddly American usage, I think.