There are several factors that come into play when you ask about veneers on the front teeth chipping or coming off. This is, in my opinion, dependent on how ones bite is prior to veneers being placed and how the dentists designs the case. For example, if you are getting veneers because you have ground down your teeth or have numerous chips on your front teeth from the way your teeth meet together, then you have a bite problem.
Why would you have veneers placed to the same position that you ruined your natural teeth? Many times there are other problems such as TMJ, airway or sleep apnea issues, or occusal (bite problems). These should be corrected prior to veneers being placed. If you are not willing to correct these issues, then a nightguard is indicated for nightime use. Often veneers can be used to correct certain types of "end to end" bites as well.
Another factor is the design of the veneers and how they are placed as well as the materials used. One of the new materials, Emax, is exceptionally strong and can be fabricated to .3mm thick in the hands of a skilled dental ceramist. In the past, certain materials could be thin but were not very esthetic as the shades of porcelain could not be layered.
In short, porcelain veneers can last a long time and be relatively maintenance free as when correctly placed and designed. They have truly helped change the smiles and lives of so many patients.