I am by no means an expert on horses or rescues, but since I've been riding and volunteering at a rescue for the last two years and I'm seeing so much negativity I feel the need to add my two cents.
The rescue that I ride and volunteer at is a 501c3 non-profit orgnaization. The rescue horses from animal control, owners who voluntarily donate them and other sources. Their goal is to get the horses healthy and help them find good families.
While they do seek donations and volunteers, most of the food, vet care, etc. that the horses need is not always covered by donations. They offer lessons, trail rides, pony rides, farm tours, summer camp and whatever else the owner can think of to raise funds.
I have seen horses come into this ranch starved to the point where they are skin and bones. We had a thoroughbred come in three weeks ago that had a tumor on his back leg the size of a softball. We have three horses that are blind in one eye and one that is completely blind. I have seen these horses given an amazing second chance and I have had the opportunity to care for, work with and in most cases ride many of them.
A great many are adopted to good home. The fee that is charged for adoptions in most cases doesn't even scratch the surface as far as what has been spent on the horse. The contract that is signed does state that if the adoptee can no longer keep the horse, they must return it to the rescue. We have had several horses returned over the time I have been there, one of my favorites was returned twice (once because they could not longer afford him and then cuz the lady who adopted him didn't listen when we told her what he liked and didn't like and she got her butt thrown). There was one case recently where the adoptee no longer wanted her horse and we were full up...but the owner of the rescue went above and beyond to help the adoptee find a new home for the horse.
The horse that I lease now, is a four year old Paint who was rescued when he was only six months old. He has a scar across his nose from the halter that his old owner left on him and he snagged it and cut his nose up. He is a character, but he is my buddy and without the work they do at my ranch he wouldn't be here.
If you want to adopt a rescue, spend some time at the facility, volunteer, ride the horse you are interested in. If the rescue is 'legitimate' they will welcome you to come and check them out, since they want to know the people who are adopting the horses as well.