Former unicycle rider, wrestler, Judoka and boxer here. I can confirm that learning how to ride a unicyle did not meaningfully improve or impact my grappling or takedown defense.
When I was stationed in the middle east, in the evenings I used to grapple two or three times a week open mat with other grapplers. We would start on the feet and go for takedowns to submission.
I used to ride a bicycle about a mile across the compound to the gym tent. Out of boredom, I started riding the bike with no hands on the handlebars for long stretches but one evening, an MP stopped me and said that wasn't allowed on base. I looked it up in the base regulations and sure enough, it was required to have both hands on the handlebars when riding a bicycle.
That night I ordered a unicycle. It took a month for it to arrive and clear customs but I spent 2 weeks teaching myself to ride it, about 20-30 min/evening. Then I made a point to ride it to the gym, in view of the MPs and they couldn't say anything because there were no handlebars. Moral victory complete.
But that thing was a pain in the ass. Takes a lot of hip and thigh work to maintain balance as you're pedaling to keep yourself stabilized, especially when changing direction or speed. Way harder to balance than on a bike. For sure it had no impact on my TD defense but if anything, might have improved my guard work because you have to control and balance the seat with your hips just like you might control an opponent in guard.
But I didn't play much guard in those days and I still think you'd be better served just drilling guard work if that's your intent.
