Yeah, the problem with trying to steer two front wheels with out a pivot is that one has to travel a little further around a corner and so sets up a 'push' against the other. Not a real stability enhancing condition.
But it's pretty darn cool, and answers a need. In my book that makes it a good vehicle...
They have longer two wheeled cargo bikes here (where the front wheel is much further away and the bucket is in the middle. It is more stable for turns, but at low speeds is much more of a pain to balance! Plus, the bucket tends to be smaller and better balanced load-wise (tricky when it is kids.... easier when it is boxes!).