The primary disadvantage of crowns is the irreversible prep work it takes to prepare the tooth/teeth for the crown to be installed. The teeth need to be ground and shaped right for the crown, which cannot be undone. The other disadvantage is the cost compared to possible other options for tooth repair
Advantages of the crown are stability, cosmetic appearance in the mouth, and longevity. A porcelain crown can look better than real perfect teeth, and if there is tooth decay or a missing tooth the change in appearance will be striking and dramatic, but look completely normal and healthy.