My educated guess is detecting humans/children in places where they shouldn't be. Sending a tractor across a field controlling everything is easy - tractors have been doing this for more than 25 years. Detecting something that shouldn't be there and not killing it is hard, which is why all tractors that do this have some form of sensor to ensure someone is in the tractor. There are a few other issues similar to the above.

I work for John Deere, I can't speak for the company.