My dentists take on tooth whitening was something along the lines of:

Whiter teeth don't indicate healthier teeth. Genetics and exposure to certain antibiotics can permanently stain teeth and these sorts of stains don't usually respond to bleaching. They generally require veneers.

Whitening usually weakens the enamel of teeth to some extent. If you have very strong teeth enamel it may not make a significant difference. If your enamel isn't the strongest due to wear, genetics of antibiotics exposure bleaching can dangerously weaken them.

Most OTC bleaching systems are too weak to show much benefit if not overused. Overuse can damage enamel and gums. Sometimes doing enough damage to cause the destruction or loss of teeth.

Many people who use OTC bleaching agents would be better off having their teeth professionally cleaned and then evaluated before they tried to bleach them with OTC products.