For a painted door, my material of choice is MDF, or medium density fiberboard. Why? 1. The panels stay flatter than a wood door 2. Moisture-resistant MDF withstands water much better than a wood door, which makes it ideal for bathrooms and wet areas in the kitchen. In theory, homeowners should like the material better too, because it's cheaper than a wood door.
Which brings me to the kitchen job. The homeowner wanted a wood door, with a solid wood panel. Doors with solid wood panels can be problematic, as wood changes size according to humidity, and this can cause the door to warp. If the panel is wood, it will expand and contract according to the moisture content of the air. If the panel shrinks too much, it will expose unpainted parts of the door.
Modern materials like MDF and synthetic stone often get a reputation as being lower-quality than the natural materials they are intended to replace. Properly used, however, they are usually superior. When a potential client tells me they want "all wood," it's often not the wood they want, but a high quality product. And sometimes that means MDF, not wood!