Most enquiries start with a picture rather than a material, which is the right way round — what the piece has to do usually decides what it should be made of.
Metal carries weight over long spans and takes a hard-wearing coated finish, so it suits staircases, railings, gates and frames. Wood is warmer to the touch and ages into something better than it started, which is why it belongs on surfaces people use every day. MDF takes a flawless painted or lacquered finish and holds a precise shape, so it is the honest choice for large flat fronts and built-in units.
Plenty of pieces use two of the three. A metal frame with a wooden top is a common request, and there is no premium on mixing them when everything is made in the same workshop.