All-porcelain is generally not as strong as porcelain fused to metal. It has to be bonded to the tooth in order to have adequate strength for oral function. The bonding technique is very demanding and is not fully taught in dental schools.
With porcelain fused to metal, the porcelain has to be opaque in order to block out the appearance of the metal underneath. They all also eventually develop an unsightly dark line at the margin where the edge of the crown meets the tooth.
Some of the all-ceramic systems that have an inner ceramic core with an outer layer of porcelain baked on require more tooth reduction. Grinding away more of the tooth is often not desirable.
Some of the ceramic materials that are very tough and fracture resistant are also quite abrasive against the opposing teeth. Of the crowns listed above, the Empress is the kindest to the teeth it chews against.
