Not a UJM, but I did 9000 miles around the US on a 1979 Triumph Bonneville special.
The only problem I had was a flat front tire at the Grand Canyon.
The bike was not new, and was well abused by the time of the trip.
Good ergonomics for me, nice thick seat, hard fender and grab rail and a flat seat gave plenty of room to strap stuff on the bike.
If you do not want to spend any money, or have ugly heavy crap all over your bike, I do not see a way to carry a lot of stuff on most modern bikes.
And forget about 2 up.
I was once on a long ride with a friend and his girlfrend, bike camping, and when he got a flat, I had my cargo, his cargo, and his girl on my bike, while he rode his bike with a flat to the campground 50 miles away.
Once you set up camp, the bike looked like a normal bike, no hard bags and other junk on the bike.
A valve adjustment was a 10 minute job, the bike held plenty of oil, had a center stand that made fixing a flat easy, the bike was about 400 pounds, so it did ok in the dirt.
My current TU250 is a small version of that, with an over the tail light rack on the back, I can have cargo and the wife on the bike and its comfortable.
Most motorcycles have gone from basic useful transportation to solo race bikes, at least in style, or giant touring sleds.
I would guess the klr650 comes close to an all around bike, but its a water cooled top heavy pig with low power output and oil use problems.