#4 His heel-run was forgettable

Dean Ambrose has been a heel during his run in the WWE. This dates back to his debut on the main roster and the Shield's initial run in 2013.
After Dean Ambrose arrived with the Shield and helped CM Punk win, he was portrayed as the leader of the Shield and the next big thing. However, his heel run was disastrous. Even though the Shield as a heel was great, Dean's heel run was not. His reign as United States Champion during that time was also lacklustre and one of the worst title reigns in history.
Cody has no chance against John Cena? Details HERE
This simply proves that Dean Ambrose was neither great nor interesting as a heel. Even if he turns heel as a singles competitor, I don't think that he might be that interesting that he is as a face.