If I were ever on a murder trial for a person with an IQ of 66 (Texas has executed people at this IQ level) it would be virtually impossible for me to rule out reasonable doubt.
On the other hand, most of these cases probably never went to trial. The defendants probably had legal aid attorneys, and the police interrogators probably had an easy time manipulating confessions out of them--and probably no one ever looked any further.
(Note all the "probably's." Yes, some of you may have more faith in the justice systems of states that execute the mentally handicapped than I do--and you are welcome to prove to me such scenarios never happened. I've seen lots of anecdotal evidence.)