I regularly do long races like marathons and 20 mile trail races and have never had to charge my watch during those events and I am often even streaming music/podcasts from the watch. With an Ultra you should be able to easily do a 12 hour hike.
From apple website: Apple Watch Ultra Workout
Up to 18 hours workout tracking Up to 25 hours Extended Workout tracking Up to 45 hours Max Extended Workout tracking
My experience with the none ultra is that the battery life estimates are spot on.
In the end, what I am arguing is that Apple provides a different set of features and provides battery life in the Ultra that covers the needs of most people going outdoors for multiple days. For everyone else, there are plenty of other options available.