Personally, I would keep them contained in a fenced area most of the time, and only let them out when you are available to supervise.
Yes, they will eat some plants in your garden.
They are not likely to eat much from the rhubarb and potato plants, as those are "poisonous" to chickens and apparently taste bad as well, but they will take a few nibbles and scratch around the base of the plants. The scratching may be bad for the plants.
Depending on what other kinds of plants you have in your garden, they may eat a lot of them.
Your newly planted flower beds will probably get scratched up quite badly.
Yes, they may get eaten by a fox, or run over by a car.
For "running away" the answer is both yes and no. They will probably wander off your property and get on the neighbors' properties, where they will scratch things up, poop, and eat plants. But if they are already used to sleeping at your place, they will probably come home each night to sleep, and will start each day on your property before they wander further again.