I have thought some about this though others have more fully explained this better than I have...

It is not the spirit that is an unreliable means of determining truth, it is man who is unreliable as a worthy vessel for the promptings of the spirit and the spirit of god cannot dwell in unholy temples. [Helaman 4:24; Alma 7:20-21]

“…And they saw that they had become weak, like unto their brethren, the Lamanites, and that the Spirit of the Lord did no more preserve them; yea, it had withdrawn from them because the Spirit of the Lord doth not dwell in unholy temples—”

“…I perceive that it has been made known unto you, by the testimony of his word, that he cannot walk in crooked paths; neither doth he vary from that which he hath said; neither hath he a shadow of turning from the right to the left, or from that which is right to that which is wrong; therefore, his course is one eternal round. And he doth not dwell in unholy temples; neither can filthiness or anything which is unclean be received into the kingdom of God; therefore I say unto you the time shall come, yea, and it shall be at the last day, that he who is filthy shall remain in his filthiness.”

A similar warning that Moses once made to the Israelites that I think is relevant to people who do the right outward hand-waving, but do so without cleansing the inner vessel, properly preparing themselves to either defend their faith or obtain their faith...

“Go not up, for the Lord is not among you; that ye be not smitten before your enemies. For the Amalekites and the Canaanites are there before you, and ye shall fall by the sword: because ye are turned away from the Lord, therefore the Lord will not be with you.” (see Numbers 14:40-45)

They went anyway and suffered because the Lord was not with them.