FWIW, they have never been a big seller for us. I've never made the comparison you mention, but the 10x56 illuminated can be had for $849 shipped. The optics of the IOR are not their weak point.
What are the weak points if any? I like fixed powered scopes, and have an ST-10, but don't like the fact that the USO doesn't compete with some of my bigger bell scopes in low-light...
They are great in low light. A friend actually has an older one for sale. Non-illuminated and unused, I think. I've played with it and was really impressed. All IOR glass is great. PM me if you want his e-mail address. I think he'd sell it at a reasonable price.
I can't vouch for the 10x56, but I can tell you that my 3-18x42 is fantastic in low light. I would assume that the 56mm bell and non zooming nature of the scope wouldn't make it worse.