Actually, it can be just as fast as a Trygon or any other fast critter - give it Adrenal Glands and it becomes Fleet...
The trygon has the option to deep strike and can place a marker for further reserves to arrive. It's also better suited to cc and shooting than the haruspex.
At the end of the day, I can think of better uses for my elite slots. Not saying the haruspex is terrible, I just think there are better options.