'Rainbow' IMO is Ayumi's weakest album release. I think after Ayumi peaked as a j-pop star in 2001, everyone's expectations were too high and so every release since then has been argued over for various reasons. I think with '(miss)understood', many fans felt disappointment over Ayumi borrowing Geo's songs because they felt it was 'not creative'.
Basically, you can't please everyone. Since Ayumi's been around for quite a while, it's understandable that some fans have walked away.