What are the differences between Drush 9 and 10 We are using Drupal 8.8 with Drush 9.

What are the main risks and benefits of upgrading to Drush 10?

The official release description is rather vague.
You can check the difference here https://github.com/drush-ops/drush/compare/9.7.1...10.0.0

As you can see, they are mostly fixes and improvements. 

There are also some compatibility changes:


  
  Source: https://docs.drush.org/en/master/install/


As long as you're using PHP 7.1+ there is no risk. The benefit is that it will still work once Drupal 9 comes out later this year.