After being away from Drupal work for a long time, I just got back into it, and right away ran into a problem with Drupal 8 where, after migrating a Drupal 6 site to Drupal 8, I was unable to log into the new Drupal 8 website. I have no idea what the migration process set my user password to, but it wasn’t any of the ones I used on the old Drupal 6 website or on the new Drupal 8 website — probably because the migration process zapped my Drupal User 1 account.
Long story short, instead of using drush, I decided to use the Drupal Console project to reset the password, and it worked as advertised. After installing the Console project, just type drupal list | grep password to see the command to reset the password, and then use it. At the time of this writing the command is drupal user:password:reset. I have no idea if that will change in the future, but for now you can type that command and then respond to the prompts. You’ll need the User ID for the account that you want to modify, and you can get that by looking at the Drupal 8 users and users_field_data tables.
