Purebred cats are usually quite predictable in terms of their appearance and their personality thanks to careful breeding practices. This is one of the reasons why they are so popular, because you know what you are getting even if the cat is still very young.
However, purebred cats are also more prone to inherited medical conditions due to strict and selective breeding. Mixed-breed cats usually have less health issues, but it is impossible to predict their appearance and behavior simply from looking at their parents.