I would not trust calorie calculators for activities like unicycling at all.
First, it depends on how skilled the rider is. The learning curve for the unicycle is quite steep so it takes time to be able to ride it efficiently. Second, how often does the rider mounts/dismounts? The mount is a short explosive movement (basically jumping on it), so, especially when learning a new skill, with lots of falling/mounting, it would require more energy. Third, are jumps, tricks also part of the training? They can be really intense. In contrast, riding the unicycle in a flat, straight line by an experienced rider would be low intensity, similar to jogging/running.
The conclusion is, the amount of energy required by unicycling is quite individual and depends on how exactly the person is using it, and I doubt that there is extensive research on this topic.
Therefore, I think it is better to approach this question from another point of view. What really matters is motivation and what makes you stick with an activity. Even if we knew how many calories per hour you specifically burn on the unicycle, it makes no sense to compare with something else, without knowing how much time you will actually be motivated to spend on each.
Unicycling requires skills, so it really comes down to your personality.
Are you that kind of person, who easily gets hooked on something hard, and enjoys learning difficult skills? Or do you just want to lose weight in the most efficient way with the least effort invested? Anecdotally, all unicyclists I know are slim, because they spend too much time on the unicycle and often even forget to eat when they are really hooked on learning a new trick :)