Advantages over React: Routing built in. Smaller/faster. You don't get numb to the word "dangerously" being used routinely for data that you trust.
Debatable: Mithril does not wrap native browser events. This is an advantage because what you learn about events applies outside the framework. It may also be a disadvantage; debatable.
Disadvantage: React is a more attractive keyword on your resume and in job listings.
EDIT: Forgot my example - https://medium.com/front-end-hacking/how-it-feels-to-learn-j...