Who needs to hack?
Like most tech types, we're overthinking it. People will mess with the coming horde of smart devices the way they mess with that other horde of smart things known as animals. Ever see somebody tease a dog? Suppose you come across a trundling delivery bot with collision avoidance. How many ways can you think of to interfere with it just by standing in the way, or placing a handy object in its path? Once some bright spark comes up with a way to make the bot trundle in circles from one obstacle to another, the trick will spread like wildfire.
Even the autonomous cars are going to be vulnerable. Read Clarke's "The Death Ray" to get an idea of what can be done with a bright light shining from an unexpected direction on a car. And since the autocars will also have collision avoidance, you can imagine the scenes at pedestrian crossings. ISTR that a cyclist already played some tricks on a Googmobile during road tests.
When it comes to buying, installing, maintaining and using tech, people are dumb. When it comes to abusing it, they come out ahead every time.