We went about 3 years free ranging our back yard before losing one to a goshawk. Now they don't roam because I frequently see Hawks circling. Think we just got lucky those first few years. If you do decide to let them you will do better if they are ranging under shrubs rather than on the lawn.
As for the garden, yes they make a mess and destroy stuff. I had some areas such as the vege garden that I kept them out of with some 3' plastic wire and tomato stakes round the beds so I could still reach in to pick. On my flower garden I put chicken wire down flat on the ground, cutting it in round plants I didn't want them scratching up. They would wander through and pick out the weeds, not really bothering my plants like they did the Veges. It was the scratching that did all the damage and they can't scratch where the wire is laying. A light layer of hay over the top and it wasn't very noticable. Have to admit that I'm enjoying being rid of all the wire and fences now they stay penned up instead.