I've been frustrated by Siri numerous times. For one thing, she's terrible with understanding regional accents. If you have a notable northeastern or southern American accent, you can pretty much forget using her. If you don't say things exactly the way people sound in the Bay Area of northern California, chances are she'll misunderstand you. Second, if you need to use names of things not found in the dictionary, (or pronounceable indexes of your iPhone, like your contacts, or Bing or Google) and don't know how Siri would guess at pronouncing them (often her guess is wrong, but if you say it the right way, she won't understand), then she just guesses, usually coming up with something totally incorrect and useless. Further, there's nowhere to report these problems, because Apple knows they would be totally overwhelmed by all the problem tickets people would submit.
This is especially frustrating in a context where tapping out corrections is not possible, like while driving in the car.