I did a big ride with a chap on one last year in the Black Forest in Germany. He worked in the industry and had a very customised and high-spec bike.
He flew up the hills of course, and even got up some quite technical climbs that had the rest of us pushing.
He did OK on the downs. He clearly suffered from all the extra weight and lack of maneuverability. He had a couple of falls and damaged an electrical cable which needed a very time consuming and complicated repair.
On anything but technical downs he did OK. He had to walk down some of the tricky stuff which was not an easy task with about 30kg of bike.
I’d definitely ride one if I needed to. They are a very common site now here in Switzerland on roads, cycle paths and fire roads.