I don't know what checking these lineage societies do, but I've seen some dubious genealogy on some SAR applications that got accepted. These sources are very solid for details like the applicant's birthplace and birthdate, as well as names of the applicant's parents and grandparents. However, the farther away you get from the applicant's personal experience, the less reliable the information might be. Also, some applicants might be tempted to "fudge" their ancestry in order to join the society.
Bottom line: These can be useful sources, but don't assume that the SAR has validated them. Try to find other sources to support the information you find.