Yes riding a unicycle is good exercise as it works your leg muscles and helps get your blood flowing.
Riding a unicycle is a great aerobic exercise and also helps build muscle.
You can lose a lot of weight by riding a unicycle and even strengthen your core and your legs will usually be hurting after a long unicycle ride and that is a good thing as it means it gave you a good workout.
Riding a unicycle is not dangerous as long as you know how to ride the unicycle.
If you're just starting out learning about riding a unicycle then riding a unicycle can be dangerous just like riding a bicycle can be dangerous if you don't know how to ride the bicycle.
Riding a unicycle is harder than riding a bicycle because you have to balance the unicycle more than you do a bicycle.
To learn to ride a unicycle you need to hold onto something until you get used to riding it and eventually you may be able to ride it without holding on anything.
However not everyone can ride a unicycle no matter how much they practice.
I never could get the hang of riding a unicycle but my 8 year old son can ride a unicycle.