The mobility actually makes him quite formidable. Instant mobility means you can take greater risks before being in the same danger another demigod would be.
An Erebus player can chase another demigod much deeper without being in danger than any other demigod can. That kind of risk mitigation alone is very powerful, not to mention the fact that Erebus himself can take basically any other demigod in a stand-up fight. The freedom to take more risks gives him a vast edge over other players. Mist is another mitigation skill that's absolutely amazing, you can instantly drop any debuffs, instantly avoid any incoming nuke, and stay invulnerable for a fairly long to infinitely long period of time while your batswarm finishes cooling down.
No matter how you look at it, ridiculous escapability is an enormous advantage that can and does get exploited by superior players to translate into more offensive power.
