As a former member of the Trailhead team, I am hesitant to write this, but I think it is an important thing for people to be aware of. If you have been using Trailhead for longer than a year, then you need to be aware of these pretty serious limitations.
While you can retake challenges for badges and projects, you cannot do so for Superbadges. More importantly, once you complete a badge, you have no way of knowing that the content for that badge may have been significantly updated. As in, all the content you understood when you completed the badge, is no longer relevant.
As I am sure you are aware, Salesforce does major releases three times a year. When they do, they prepare comprehensive release notes and the Trailhead authors are instructed to review these notes and update their content as necessary. That part is great. What is not great is that you as a consumer of Trailhead content have no way of knowing that the content was updated (without revisiting EVERY badge you have earned to examine the content).
The badge itself just indicates that it was completed on a certain date. As in the image below of a badge I completed in 2015. This badge has since been significantly updated and the content that was once relevant when I completed the badge is VERY different.
How do you know that you are aware of the latest features and updates to the platform? Well, it is up to you to pick through the release notes and then cross reference any Trailhead content that you may have completed. Nice, right?
Just wanted to make sure everyone was aware of this (what I consider to be) serious limitation. I hope the team is actively addressing it. I would say I know, but they don’t talk to me since I left the team.