A: Yes, all-ceramic materials degrade in strength over time - some more than others. Zirconia ceramic has an initial flexural strength of over 1200 MPa, so even if it degrades 5-10% or more, it is still significantly stronger than other ceramics.
A: Fine diamond burs can cut thru a zirconia crown just as fast as a PFM crown.
A: Weakening of colored zirconia ceramic copings has not been observed.
A: Yes, the marginal fit of crowns and bridges fabricated from zirconia copings can be as good or better than PFM crowns and bridges.
A: The thicker the cement the weaker the all-ceramic restoration. Cements are not as rigid as zirconia ceramic, so a thicker cement layer will cause more flexural deformation of a ceramic core, increasing the potential for fracture.
A: No, but the use of Rocatec/Cojet (3M ESPE) does. Silica particles are embedded into the surface of zirconia and then silanated. This treatment can increase the bond strength of a resin cement to a zirconia coping.