I would consider them a high end maker. Looking at watches like the Freak, the Sonata (my personal favourite from the brand), and other amazing watches they have made, I don't think you could not consider their work high end. They do have lesser models, mostly dive watches and many using an ETA movement with a module on top. Those are less high end, but by all accounts still nicely made watches. The old 1846 Marine Chronometer model was one that I loved the looks of, and it if wasn't modular, I would probably have bought one. I remember myself and 2 buddies all going to a store once where the had 3 of their divers in different colours, and we remarked that we all liked a different version. We nearly walked out with our own version of that watch...
I see allot of Yachts and large vessels ( from cruise ships to bulk cargo tankers) in my work and notice several of the more expensive yacht owners and ship Master's seem to have Ulysse Nardin,s. They have a nautical following that would probably outweigh most other brands.
I believe the nautical following. I think their designs require a certain taste, I've looked at the brand often and never really found a model that sung to me...
I like their Marine Manufacture but you should keep it forewer, because UN resale prices are very low.
High end. I have been tempted by many a vintage UN, but they seem to be among the most re-dialed brands in the vintage world. And even when I'm pretty confident it's not a re-dial, I'm usually not confident enough, because that boring "CHRONOMETER CO" font is so eminently imitable.