I own a UMI King Silver Flair 2055. It was my first step-up horn, a Christmas gift from my parents. It served me well throughout high school and college, and I still play it. In fact, I took it out on a gig last weekend. I have also had the good fortune to have a couple of vintage Silver Flairs come through my stable- one of them a KMI, and the other one an H.N. White. Both were outstanding horns. Both very different from their UMI cousin.
As with any large-scale manufacturing process, quality can come into question at times. Bad quality, even if sporadic, brings bad reputation. Perhaps I was lucky to get my hands on a truly great 2055 Flair. I guess I wouldn't call it "hate" on UMI, but healthy skepticism of the quality of their products. As with anything, however, there are exceptions to the rule of public opinion.