"While the system works with most US games, individuals have reported problems with a few (primarily Castlevania III, Gauntlet, and Mindscape games) due to special in-cartridge hardware being incompatible with the NES-on-a-Chip ASIC.
There are also serious audio reproduction inaccuracies which cause music and sound effects in most games to play incorrectly or not at all. In Dragon Warrior III, some of the bass sounds are dulled, but the rest of the sounds are unaffected. The model of NES-on-a-Chip used in the system provides comparatively inferior audio compared to many other Famicom clone systems.
The system also suffers from particularly serious slowdown issues evident in stressful games. These audio and slowdown issues are particularly apparent in games like Kirby's Adventure which feature stressful CPU operations and complex audio."
I'd rather spend the extra money on a real NES or Famicom than waste $20 on that crap.