It depends on your dentist and how much tooth is left when the drilling stops. They are very strong when your dentist is conservative with the drill and leaves the majority of enamel in place. Feldspathic porcelain bonds extremely well to enamel. The tooth is actually strengthened by the presence of the new veneer.
The feldspathic restoration is considered monolithic with one type of porcelain. It does not have a strong coping substructure to support it. The tensile strength of 85 MPa is low on its own and needs to be treated like fine china until bonded onto the tooth. This type of restoration gains its strength from the bond to enamel. If supported and bonded to enamel, it is incredibly strong.
Most dentists drill too much tooth structure away for this type of veneer. Often less than 50% of the remaining exposed clinical crown has enamel to bond to. If this is the case, then the feldspathic porcelain restoration will be one of the weakest options. It would be contraindicated for patients who are tough on their natural teeth. Understandably, the choice of restorative material is often made intra-operatively. When more than 50% of the enamel has been removed during the drilling process, a crown might be a better option. Crowns don’t require as much enamel for bonding.