Take one of the world's most powerful telepaths, join her with a practically all-powerful cosmic entity, and it's safe to say you're in for a pretty powerful superhero - albeit a pretty volatile one, too.
With the Phoenix's powers, Jean - who is, again, one of the world's most powerful mutants - suddenly becomes more of a force of nature than any kind of mortal figure. Everything from astral projection, reality-bending, and of course energy manipulation are available to the Phoenix, and they're available on a scale that is never not terrifying to even just see on a page.
Importantly, while the two are joined, Jean cannot technically be killed, as instead of dying the Phoenix Force forms an egg that the completely healed Jean returns from. This means not only is Jean totally overpowered in this form, but she also is unkillable, so no plan to defeat her will leave her that way for long.