I did the research to get my mother into the DAR. I found some distant cousins of hers who had joined the DAR under their common ancestor, and she joined under that ancestor. I now think that the records that were accepted for that ancestor's Revolutionary War service were for a different man of the same name, not even related to our ancestor (unless related way back in England) and that our ancestor was living in a different state at the time of the Revolutionary War.
The first person to join the DAR under that ancestor was a prominent lady and they may have been less inclined to scrutinize the evidence too closely in her case--but I think there were many cases of sloppy research in the early years. My understanding is that they are now much stricter in their standards of proof.
I know the SAR is very demanding in requiring proof of the ancestor's service and in proving descent every generation down to the person who wants to join.