I've had two new UX50s, one just after its US release, the other about a month ago. They both worked fine, never had any problems, except that the screen always seems very slightly crooked when you close it. In other words, you can see it's not exactly parallel with the buttons/jog dial on the bottom half. But there were no real problems with either of them.
I don't think the UX has sold anywhere nearly as well as the TH55. The first week the TH came out in Japan I think it accounted for 60 or 80% of all PDA sales surveyed, it was sold out in Japan, I noticed it sold out very fast in the US too, both online and at my local stores.
I suspect there's a disappointment factor - people find the UX to not be what they thought it'd be, so they just return perfectly working units and get their money back. The UX is kind of unusual in its combination of features, and I've seen a disproportionate number of people complain about the things it doesn't have.