If you get one of those USB tuner sticks, then there's the issue of overheating. Had this problem on an HVR-950. After 2 hours watching on the laptop, I was getting stutters. Apparently, unit was running too hot. Tuner sticks are also unlikely to have a hardware MPEG2 encoder.
Otherwise, if you get a relatively big USB tuner like the HVR-1950, I don't think you'll run into any problems related to USB.