Not just UZs...all camaras...including DSLRs do not auto-focus as well in low-light/low-contrast situations. Auto focus is not "magic". When in passive mode, auto focus in most P&S cameras works by contrast detection. Low light=low contrast=AF difficulty. It's that simple. In active mode (when the focus assist comes on) they attempt to focus on the reflected light or bounced ultrasonic waves (depending on the camera)...not very accurate. Most DSLRs depend on phase detection, which is directly related to the amount of light available...
But, the real message here is: The best way to have good experiences is to be informed and have reasonable expectations!
Some Gear: Nikon D700; Nikkor AF-S 50 f/1.4 G; Nikkor AF-S 24-85 3.f/5-4.5 G ED; Tamron 28-300 f/3.5-6.3 VC; Nikon SB-800; Velbon Maxi-F; Canon Pixma Pro 9000; Canon S3IS, Canon SD500; Epson 4990; Epson P5000; Wacom Intuos 3
Main Software: Capture NX2; Adobe PhotoShop CS2; Corel Paintshop Pro X2 Ultimate
Sold: Canon XT/350D, EF 24-105mm f/4 L IS, EF 100mm f/2.8 Macro; EF-S 10-22mm f/3.5-4.5, Sigma 18-200 OS; Canon ET EF 25II; Kenko Pro 300 DG, Canon 430EX, Canon BG-E3.