My experience with bantams is mostly positive, but here's a list of what I've dealt with in them in no particular order:
- They eat almost nothing, happily picking up the tiny scraps that the big chickens don't want to be bothered with
- Typically very good layers and enthusiastic broodies
- Typically very good mamas
- Typically very spastic, high-strung, vocal, spookable, and good flyers; though I never hand-raised one. They were always broody-raised or taken in as adults, so that makes a difference
- A little bit more frail and more prone to 'accidents' (getting stepped on, injured, trapped under things, or in one case, blowing out of a tree in a windstorm in the night and drowning in the trough despite having an 'escape' ramp in the trough)
- Excellent and enthusiastic foragers
As far as the garden, they're still chickens, they still dig. I personally wouldn't do it. Even if you introduced the adult bantams into a fully-established garden they'll be uprooting carrots and potatoes, ripping squash leaves apart, eating your tomatoes and leafy greens, and plucking up every last sprout that crops up.
The ONLY chickens I keep in the garden are chicks 2-6 weeks old. Once their head feathers come in I kick them out! That's when they start digging and ripping sprouts out! But I like to brood the chicks in the garden for pest control.
Edit; I do have a friend that runs chickens in her garden. She focuses mostly on tomatoes; her toms are in 2-3 high stacks of tires that she fills with poopy-chicken-coop-bedding. So the birds can't get at the
roots or ground around the toms and they're a couple feet above the ground. She also cages them with 2x4" welded wire fencing and protects them when they're babies. All of her other garden stuffs is started in a
greenhouse and transplanted into chicken wire or fencing cages to protect them from the birds. Zucchinis are likewise planted in stacked tires and trellised plants have a cage around the bottom of the
trellis. She only loses a small amount to the birds.