I have rescued a handful of horses over my lifetime.
My current rescue is a registered Arabian who now 23-1/2 years old. He was a head attached to a skeleton when I rescued him as 7 year old.
It also didn't take me long to find out he also had an injured vertebra, which meant I could never ride for the type of trail riding I used to do. He ended up being a lesson horse for children 12 and under and if they weighed less than 100 pounds.
He is the absolute best babies and toddlers babysitter I have ever been privileged to feed hay to. He is the passive leader in my herd of four and his little 13.3H self doesn't have a bit of problem putting the 16H, 3rd in line, TWH in his place if needed
Adopting is, indeed, a magnamonious effort. However, as the previous posted commented all rescue horses can come with some underlying and/or unforeseen issues. Those issues may need addressed immediately or later in the horse's life. Folks with little-to-no horse experience are often unprepared emotionally or financially to deal with those issues.
Anytime horse adoption is being considered, folks should try and learn as much as possible about the horse and then be prepared to "double that" once they get the horse home because there is always some issue that will crop up later that no one knew about.
I spend a LOT of money to keep my little Arab healthy - always have and will until the day it's his time to meet his ancestors. Many folks are not able to devote a lot of time and money, so it's easy to get in over their heads when they are only trying to do the right thing