Reshiram

Reshiram possesses a good offensive typing, great offensive stats, and an acceptable Speed tier, making it an extremely potent wallbreaker. It takes advantage of the prevalence of defensive Steel-types to blow holes through foes. However, Reshiram's typing is often a detriment defensively. Reshiram is susceptible to all entry hazards and weak to Stealth Rock, making it extremely easy to chip down. Its neutrality to Water means that other Dragon-types such as Eternatus and Palkia can fit onto teams much more easily. Similarly, Ho-Oh can take on Xerneas's Moonblast much better, leaving Reshiram's defensive capabilities completely overshadowed. All in all, Reshiram's biggest problem is the opportunity cost of running it on a team.