Potential interference, crowded spectrum, battery required, extra weight due to the presence of battery, and if the encryption is weak then you have a signal broadcasting all that you type in the vicinity which could be a security nightmare.
I didn't realize people had such low expectations for "just works". Getting a reliable signal for such a short range and low bandwidth is obviously possible, as is using proper encryption. Batteries can be light, with the weight compensated for, and charge wirelessly.
The reason I said "just works" was to avoid an exhaustive list of issues that have already been solved in other products, or could be solved with some effort.
