I'm happy with my chickens and I think the good outweighs the bad, but since you asked:
They're destructive. Chickens are always scratching at the ground and picking through ground litter which means you'll have to devise a plan to keep them out of your garden until the plants are established.
Hens have to mature for about four months or longer before they begin laying eggs. That means paying to feed something that isn't giving you anything in return. Hens continue to live long after they'd stopped laying eggs. That leaves you with the choice of culling the chicken or continuing to feed something that isn't giving anything back. The decision to cull the chicken is tougher than you might think. At that point you've likely had the hen for a number of years, and she's been a faithful contributor to your kitchen table. Your kiddos might be fond of the hen. There's a fine line between a farm animal and a pet.