How to ensure successful update after system failure during updb causing the site to remain in maintenance mode?
When running subsequent drush updb that says &quot;no updates required&quot; does this guarantee the previous updates were successful.

When Drush queues up batch items to be processed for drush updb, it adds these operations:

Any module schema updates
Any entity definition updates (off by default, not supported after 8.7)
A cache clear &amp; post hook update functions (on by default)
Toggle off maintenance mode (if it not already enabled)

If the only updates were module schema updates then you might be in a scenario where things are ok; there are no post-update hooks needed (e.g. some batch data ops following a schema change) and only the last step of toggling off maintenance mode didn't complete.

How do I ensure the integrity of the system when this happens without restoring a backup and running the updates again?

You'd have to audit the list of updated modules to double-check each schema update was applied and there were no post-update hooks missed in the update.
Depending on your update scenario this may or may not be easy, hence the best practice is doing a DB rollback and re-applying updates to ensure data integrity when uncertain.