In the process of upgrading sites from Drupal 6 to Drupal 8 using the information provided by Drupal Documentation, using the drush implementation.
I have a multisite setup in Drupal 8 and found that drush reads the current site according to the values found in /sites/default/settings.php so for each site I upgraded I had to change the database settings in that file to suit the site I was working on.
Amazingly all the content migrated without a problem. The taxonomy also migrated but did not display. After investigation I found that the taxonomy_term_hierarchy table is essential and that this was empty. Since all my taxonomies are flat I was able to rectify this with a simple bit of code run in devel/php
$result = db_query('SELECT tid FROM {taxonomy_term_data} ')->fetchAll();
foreach ($result as $record) {
db_query('INSERT INTO {taxonomy_term_hierarchy} (tid, parent) VALUES ( :tid, 0 )', array(':tid' => $record->tid));
}