I have had 15 hanging baskets in my backyard with impatiens for the last 20 years. Always beautiful! This year they are all being destroyed, time and time again. Some look like they've been sat on and some eaten on. I don't see any slugs in the pots ever. I have squirrels, but they have never before done that much damage. I've sprayed them with poison, squirrel repellent, used cayenne pepper, nothing seems to help.
Slugs could be some of the damage, and they are rarely seen, since they come in the night to devour plants.
You can sometimes see the silvery trail from them left behind.
Rodents sound like a likely pest with the damage that you mention.
This could be mice or even squirrels.
Here are some links with more information.