• Have an unattractive shape
• Do not feel comfortable
• Interfere with chewing
• Fall off
In most cases, these can be corrected simply by a skilled cosmetic dentist. Many porcelain veneers that people consider unnatural are what we describe as "no-prep" veneers that are simply bonded to the surface of your teeth. They have to be opaque to attempt to mask the color of your natural tooth enamel beneath, and they add additional bulk to your teeth, making them look blocky and unattractive. A cosmetic dentist can remove these and replace them with more natural-looking veneers that are used to replace some of your natural tooth enamel and can have more natural-looking color, translucency, and shape.
No-prep veneers are similarly often to blame for being uncomfortable or nonfunctional. Because they just sit on the surface of your natural teeth, they build up your teeth in ways that can interfere with your bite.
It is rare for veneers to fall off, but it does happen. It's more likely to happen if you have veneers bonded to dentin as opposed to enamel. It is more common (though still relatively uncommon) for veneers to fail by breaking. This is more likely if the veneer has been applied to a dead tooth, or if you have unresolved bite issues, such as tooth clenching.
If you have a veneered dead tooth, and the veneer keeps failing, it may be necessary to remove the tooth and replace it with a dental implant to get the attractive appearance you desire. For bite issues, a cosmetic dentist trained in neuromuscular dentistry can help you resolve the bite issues so your veneers last for their expected life, about 10 years.