Yumの依存関係解決アルゴリズムが壊れている、と言っている
https://rpm-software-management.github.io/page7/
The reason of initiating DNF project was because of the biggest three pitfalls of Yum:
undocumented API, broken dependency solving algorithm and inability to refactor internal functions.
The last mentioned issue is connected with the lack of documentation.
Yum plugins are using any method from Yum code base thus any change there would cause the sudden crash of the Yum utility.

http://www.linux-magazine.com/Online/Features/Will-DNF-Replace-Yum/(language)/eng-US
YumはAPIのドキュメントがないので内部関数すべてがPublic API状態
リファクタリングもできない、不要なものの削除もできない
メンテナンスもテストも難しく、新機能の作成も遅くなる