That is a myth. In the absence of recorded information, a robust RA methodology forces SMEs to make careful and informed estimates based on their experience and judgement. From direct estimates of real-world phenomena, reasonable risk estimates emerge, pending the acquisition of better data. Therefore, I would respond that lack of information should drive you towards a more robust methodology. Using a lesser RA approach with a small amount of data just compounds the inaccuracies and does not improve understanding of risk.