PFM is porcelain fused to metal they have good strength but the porcelain can chip if you bit on something very hard or it's a very old crown and the metal gets exposed. So I would advice zirconia/ Emax crowns the metal free options.They provide good strength and aesthetics.

Brittleness is a definite physical property of any porcelain whether used for dental purpose or otherwise. It is because of the three dimensional chemical structure of this material. It would be difficult to answer the question as to how brittle a porcelain is because it is not readily measurable entity. Yes, if the porcelain gets chipped off from a PFM cap, the underlying metal structure would definitely be exposed. But we would not say that "in this sense" an all ceramic crown is better, because if the porcelain is chipped off from an all ceramic Crown the underlying core structure of the alloy called as zirconium will also be exposed although it would not be very conspicuous because it is tooth coloured. An all ceramic crown is better because it is higher in strength and more accurate in terms of the marginal integrity. Regards.