I agree with SDS to be completely honest, especially in the case of Phoenix, without level 3 X-factor she is a completely different beast.
Now on to this:
Uuh, a level 3 X-factor Phoenix is NOT easy to deal with at all. Sure you can snap her in, but then what does she do? Jump away homing fireball all day and switch back out.
Learn to snap in in the right situations, where she can't run away, or just counter her fireballs, sure the Heavy Fireball can track, but it actually has a relatively slow startup that can be countered by a lot of characters, especially if they have a fast beam or beam super (think Magneto and Akuma). Phoenix has alright keep away, but until she is Dark Phoenix (where her fireballs are faster), it is not all that great, so any dedicated keep away character will easily win out there.
The final piece of evidence that Phoenix alone is not that great has to be the grand final of Wednesday Night Fights last week, where Justin Wong and Floe made fools of themselves trying to use Phoenix on a whim. Anyone can pick up Sentinel and learn his fairly simple combos and abuse his armoured moves to gain a pretty good effect, but Phoenix is actually a lot harder to use. Sure if you put in the time and actually learn to use her and dedicate a team to that, she will seem a little broken (especially with level 3 x-factor, which is whole other thing), but she is nowhere near a level which would require a nerf, which is not to say that Sentinel was either, though everything I have seen has shown that Sent has not really changed that much, you just have to be a little more careful.
Also a random little nitpick, unless forced to, or in a situation where it is unbelievably beneficial, no Phoenix player will activate x-factor until after the transformation, because Dark Phoenix needs it to recover the red life she inflicts on herself.