User blog comment:EmpyreanSmoke/Supernatural Q&A/@comment-79.115.68.194-20150918120536/@comment-24834479-20150918233732

The stronger the vessel is, the more powers the Angel is able to channel and use. There is a single family line (starting from Cain and/or Abel), which holds the purest, strongest vessels. Sam and Dean are in this direct family line. As this family line branches off into other lines, these new families are growing weaker and weaker vessels. The further the human is from this direct family line, the weaker he is as a vessel. Hope this helps.

Adam is still a very pure vessel, but Michael is not as powerful within him as he would be in Dean. Lucifer definitely has more of a chance at beating him than if he were inside Dean.