How to repair "Mismatched entity and/or field definition" when "drush entity-updates" fails?
Although this question is quite old and no longer relevant for Drupal 8 as drush no longer has entity-updates, I believe people would still be landing on this page with Pending Entity Updates messages in the Status Report. I ran into the same scenario and wanted to address that issue. 

The message is shown when the definition of the entity in the database is different from the definition of the entity in the file because of deleting or adding a field in the entity. That is something that is going to happen a lot when you are developing the module and the entity definition keeps evolving. 

Uninstalling and installing the module will solve the problem because you are recreating the entity again. 

I ran into this same problem and thought I should get to the bottom of this as it was not possible to keep uninstalling and installing the module. 

Drush used to have entity-updates as a feature that would allow entity definitions to be updated on the fly. However this was removed from Drupal core in 8.7.0 (Support for entity updates removed from core)

The work around is to install Devel Entity Updates module and run the dentup alternative

If you are using composer

composer require --dev drupal/devel_entity_updates

drush en devel_entity_updates

drush dentup

That should address this problem. The devel entity updates module is to be used in develpment environments only and in higher environments you are expected to push changes to your entities through hook_updates