Old big IMF's used to use walnut grained formica and looked good until excessive sunlight caused the edges to lift

Nowt wrong with MDF as long as the designer knows what it does and can deal with it accordingly. I believe Harbeth now use it - thin and with copious added damping - and their boxes are incredibly "dead" and with just a tiny amount of resonance at 900Hz or so, well down in level and around the modest distortion levels I believe. Speakers are the worst and most distorted part of any HiFi system, together with the listening room, so do what you need to do and do it carefully and well