I have some experience with rescues. It's like OTTBs. It's a gamble, really. You can find some horses that were good, trained horses, that just went into a neglected home. They might not have been hurt or abused, just terribly underweight and seized for that reason. I've read a few of those horses, and besides having some weight and grooming problems, they were the sweetest things.
I wouldn't recommend it, no. However that doesn't mean there aren't some surprising finds. Habitat for Horses is a dreadful place, however there are some horse rescue locations around here that train their horses and get them back to relatively normal. There are many places that will let you test ride the horses, too. Just because a horse is old and trained doesn't mean they can't give you hell. I think it has more to do with the individual horse.
Just my two cents.