The BATFE has taken the position that a medical MJ card is an indication that the holder is the user of an unlawful substance and, as a result, an FFL dealer may not knowingly sell to such a person. I am not aware of any cases of the feds maintaining a database of medical cards for NICS purposes, but I have read that holding a medical MJ card in CO dsqualfies one for a CO CCW permit.
The feds standard is "current user", so a long past MJ conviction does not lock one in as a PP person unless a felony or > 2 year misdemeanor.
Curiously, MGL *prohibits* issuance of an LTC to anyone with a MJ criminal conviction, but concurrently prohibits the use of a civil MJ citation against an individual in any licensing determination or state benefit. As a result, the *identical* action can either make one a PP, or be something that cannot be used against the applicant, depending on when it happened.
Yes, there is still suitability. A LE agency can deny an applicant for a reputaiton for drug use, but may not consider a MJ citation in making the licensing decision.