The first is that too much of a good thing can quickly turn into a very bad thing. Think of it like a Hawaiian shirt. You can get away with wearing your favorite Hawaiian shirt and some khaki pants, but wearing your favorite Hawaiian shirt with Hawaiian pants, socks and hat is just wrong and the awesomeness of your favorite Hawaiian shirt gets lost amongst the rest of your Hawaiian attire. Doors are similar. Too much of one wood species in one room can be just plain wrong. Instead of overwhelming a room with too much of one type of wood, make your cabinets or doors a focal point by using contrasting accents.
The second reason to steer away from getting too Matchy Matchy is that it can get expensive. Doors are most commonly made from MDF, Hemlock, Mahogany, Pine or Knotty Alder. Once you start veering away from these basic choices, the prices and lead times start to climb.