The main advantage of the qualitative impact analysis is that it prioritizes the risks and identifies areas for immediate improvement in addressing the vulnerabilities. It does not provide specific quantifiable measurements of the magnitude of the impacts, therefore making a cost-analysis of any recommended controls difficult. Since it involves a consensus of export and some guesswork based on the experience of Subject Matter Experts (SME’s), it can not be easily automated.
Reference used for this question: STONEBURNER, Gary et al., NIST Special publication 800-30, Risk management Guide for Information Technology Systems, 2001 (page 23).