The Supersoul works through the interface of our intelligence.
Srila Bhaktivinoda Thakura said that some people, in the name of spiritual life, actually don't want spiritual life and actually want to e cheated. Bogus gurus are necessary for those who don't want a real guru; they are looking for a guru but want their spiritual life to be cheap.
Sometimes devotees who had a guru who later deviated from pure devotional service are told that they chose such a guru because they were not sincere. But when devotees come to ISKCON, they get the shelter of Srila Prabhupada, so how can we say they were not sincere if they are still in the sanga of Prabhupada and his followers?
Generally, it is not our job to evaluate who is sincere of insincere.