The way I see it, the Phoenix has always had pretty ill-defined powers that are subject to very loose interpretation, so that while Jean's power can be scaled up to take on almost any foe, they are also unreliable and prone to backfiring, in which case she doesn't stand much of a chance given her slight build and limited fighting ability. This is useful for story purposes, since she will often find herself trapped and in dire peril, but always with the ability to pull herself out when her powers flare up again later in the story.