Even if the devices update themselves automatically, the trend so far has been that they don't continue to operate normally while the update is taking place. This is probably because it's difficult to replace files while they're in use by a running process.
So imagine all the things in your life stop working temporarily. It doesn't have to be all a the same time to be a problem, in fact if it's not all at the same time it could be a bigger problem. With enough smart devices updating themselves, the chances of one updating after another becomes greater, and the longer the chain of successive devices updating themselves is the more of an intrusion into your life it becomes.