Dark Phoenix requires and consumes all 5 hyper bars upon Phoenix's death. As far as I know it doesn't require a command input. One downside was accounted for, as it seems her diminishing health affect was negated by the X-Factor regenerative property.

The other downside is Dark Phoenix takes a massive amount of damage. Even with X-Factor, she was almost utterly destroyed by Cap's hyper. It becomes an odd balance between playing keep-away with the X-Factor, and rushing down Dark Phoenix, that causes an unusual circumstance that could be difficult to combat.

Dark Phoenix will likely be useful early on, but once people figure out how to fight her, it won't be worth the 5 bars.