I think that "uniquely" American usage of underwhelm works, but I think that overwhelmed can be more than just an overload of expectations realized, etc. Like when one is dealing with a lot of work and deadlines and such, they experience a feeling of being overwhelmed with pressure and responsibility. I think that underwhelm could also apply to someone who is wanting of more responsibility/work... it can express their displeasure with having so little responsibility.

Ex. Sarah's children have finally grown up and left the house. She now feels underwhelmed with the work left to do around the house. She misses her tiresome chores and nagging dilemmas.

Thank you for the nice welcome. I was just told about this site a week or so ago, and I like what I see so far.