The important question here (and maybe it has been answered already and I just don't know it) is "Can you use the tablet/gamepad while it is charging?"
Because if not, this is going to be a disaster. Wii remotes were already pretty bad, where the only rechargeable batteries I'm aware of require contact to charge, meaning you can't just run a wire to the controller, and its not practical to play while its charging. This is perhaps understandable, since a wire would be unwieldy for a motion controller, but it also meant you had to have a spare already charged at all times, or you would be SOL if it died. Luckily the battery life wasn't too bad, otherwise it would have been impractical for party settings without spending a good chunk of change on replacing batteries.
If I can plug this in and play while charging, then I don't particularly care what the battery life is. But if I can't do that, then this is going to be a real problem for a system that I'm realistically not going to have a spare controller.