While the USB charging adapter likely provides less power to your tablet than the bundled AC adapter, your tablet should eventually reach a full charge with either one. The exception to that would be if you're keeping your tablet busy enough while connected to the charger that all the incoming power goes to current activity rather than to charging the battery. For instance, playing media (especially streaming) at full volume and brightness can easily soak up all the juice a low power charger can provide.

Personally, I've used both a cheap knock-off and a genuine USB power adapter to charge my N800, and both easily charged my tablet fully.