Composites are not totally compatible either. Some composites are less biocompatible than others because of the amount of iron oxide, aluminum oxide, barium, and other unique materials in them. I most often use Conquest Crystal, a polycarbonate-dimethacrylate resin with a barium boron silicate filler. Direct composites can cause hairline cracks in the tooth from the hardening process, whereas indirect composites do not because they are hardened in the lab. Porcelain is more natural-looking than composites, but because it is harder and more brittle, it causes a wearing away of everything it comes into contact with and can crack instead of flex from high stress.
All porcelains contain aluminum oxide.
If metal is not used in a crown or bridge, it is significantly weaker and has an increased risk of breakage during normal function. Gold fillings, porcelain fillings, indirect composite fillings, and crowns require more tooth structure to be trimmed away than for amalgam and direct composites, and take 2 appointments rather than 1. Special order higher content gold will obviously cost more due to the cost of gold. I most often use Sterngold, E2713. Studies of gallium alloys have reported problems with corrosion, durability, tooth fracture, and tooth sensitivity.