We have a fenced area around our polytunnel which also has some raised beds 2 feet high. I built a custom duckolator that allows the ducks access out of their regular area so they can free range around the outside of the polytunnel and raised beds (they can't get up onto the beds).
Since we started doing that, the number of slugs we find around that area and inside the polytunnel has decreased dramatically, so they definitely work for pest control. In contrast, the lawn the other side of the fence can be absolutely crawling with slugs if you go out with a torch on a damp evening.
I don't think they'd bother well established plants, and they don't scratch the ground like chickens. However, they do stick their bills into and under everything and go wibblewibblewibble a lot, so I wouldn't trust them near seedlings or anything delicate.
HTH!