I've heard about it, but I know precisely one person who is on SSI for disability - my sister who has schizophrenia - and she has the mental capabilities of a ten year old.
I have an acquaintance (not a friend, I don't like him) who threw out his back at work and tried for years to get SSI disability, to no avail. He was capable of working, he just didn't want to any more. (We have a lot of call centers here in town. Indoor work with no heavy lifting.) I think all the judges in his case knew that was his goal, which is why he kept getting turned down.
TL;DR - Despite what the commercials and stories say, I think the system is working as intended.