I’ll start with a reference back to that. In my understanding, these health issues for bicyclists are almost unique to Road riders, or others who do many hours of riding per day, lots of days per week. In other words, I do not believe they show up as a problem for cyclists who ride “normal” amounts.
That said, we sit more upright on unicycles, which tends to put more weight on the “squishy” part of our crotches. We’ve all felt it. While unicycle seats have gotten better, I think our riding position is simply more difficult in terms of designing a seat that can support our weight without putting pressure in places we rather they didn’t.
It’s all anectodal, since there are no scientific studies of such things as they pertain to unicycle riders. So we ask this group, and see what pops up. I’ve been riding unicycles since 1979, some with great seats, and others with terrible ones. I remember having crotch issues on my first-ever long ride.
My friend and I decided to ride our 20" Schwinns (a Giraffe and a regular 20", with regular, 1979 Schwinn seats) to a destination 5 miles away. And back. This was an epic journey for us based on our previous experience. I had only been “really” riding for about at week at the time. I don’t know how long it took us to reach our 5-mile destination, but we both arrived with sore crotches. Peeing was accompanied by a painful, burning sensation. We ended up mostly walking on the way back, and were lucky to be happened upon by my brother, who gave us a ride the rest of the way. (I just Googled our route, and actually it was closer to 6.5 miles)
The pee-pain quickly went away, and our teenaged bodies quickly recovered. Also we stuck to shorter distances. But in the fall of 1980, we both entered a March of Dimes SuperRide event; 75 km on our geared-up Schwinn Giraffes. We were the first to start, and possibly the last to finish the 10 laps around Belle Isle (Detroit). Painful peeing again. Our advice: don’t use a Schwinn Giraffe for a 75k ride!
Fast forward: In 1982 I got a 45" big wheel unicycle, and rode it in other long charity rides, but with much less adverse effects (still a Schwinn seat). Then in the 2000s I got my first Coker. In 2008 I participated in Ride The Lobster, a huge unicycle stage race. In the tons of training beforehand, I did experience some other issues which I’ll detail below. But since those early years, with the improvement in saddles, my own choice to wear good quality bike shorts, improved riding technique and the advent of handlebars for Road riding, the ability to do very long rides with much less risk of crotch issues is definitely a reality.
Of course. But first I’d like to rule out any health issues related to falling off the unicycle. Most other sports that involve movement can lead to injury but I don’t consider that to be the topic in this thread. My broken collarbone can be discussed elsewhere.
Foot numbness:
If I ride for a long time without dismounting or changing my riding position, I will tend to get numbness in one or both feet after several miles. Some of this is probably related to shoe tightness, but I think it can also come from restricted circulation in the crotch. That’s why we sometimes call it a “circulation break” when we stop every 5-10 miles on long rides. It seems to help. Other riders have figured out to not have this issue, either by changing crotch position, moving their feet around, or just better fit to their saddles. I know of two people who have done 100-mile rides with no dismounts.
Crotch friction damage:
Usually I get a raw spot, basically at the point where my leg becomes my crotch, on extra-long rides. Extra-long is a relative term. My wife and I now do a 19-mile ride around a local lake on a regular basis. No ill effects from that. But when I go beyond what I’m used to, I usually get some chafing that needs to heal. On the other hand I’ve done some very long rides where my crotch emerged unscathed. I haven’t yet figured out the secret. 
Knee pain(s):
Over the years, I have experienced when my knees would tend to “wear out” on rides longer than what I was used to. Like the lubricating fluids in the knee joint were drying up from overuse. This would happen to me on rides under 10 miles if I wasn’t used to it, but was not a factor on some of my longest rides, presumably because I had trained appropriately and kept myself hydrated. In the 80s, riding my 45" in the annual 5-Boro Bike Tour in New York City, it seemed my right knee would always wear out towards the end, and the last several miles would be slow and painful. I think if I’d done longer training rides, this probably wouldn’t have been an issue.
I’ve experienced a different knee pain on other occasions. My first attempt to do the 72-mile ride around Lake Tahoe ended with knee pain that led me to walk the downhills, while I was still able to crank up the uphills and ride the flats. I attributed that to not using my brake enough on the earlier hills in the ride. I got about 40 miles on that ride.
A few years ago I did a century ride, and that one also ended with a very sore right knee. Why the right? Possibly because it’s my dominant leg, so it might wear out first. Possibly due to an asymmetrical riding position (as yet undiagnosed) or possibly both. Toward the end of that ride, certain movements gave me a sharp pain, and hindered my riding. It took a couple of weeks after that ride for it to go away, though I haven’t experienced anything like it since.
Absolutely what we have to work with is not scientific, and is very personal. Like saddle comfort, your mileage may vary. What I’ve learned over the years is that the human body is capable of amazing things, but going far beyond what your body is used to can definitely cause damage that you wouldn’t get if you build up to it gradually.
