Generally, it's an as-needed power and she rarely stars in her own book. Having PG get taken out on the JSA serves more of a function than restoring her. Often times, if something wounds her that much that she'd need the sundip to get battle-ready again, it's because the writer needs her off the table to highlight someone else. In her own book, were she fighting a galactic conqueror, it may come up. It's just that if she's taken out, it's time for Alan to shine and vice versa. Rarely does Clark get beat down in the JLA just to show up after a sun dip and win the day. It's usually done to let someone else be the hero of the arc.
PG is generally a team character now so while we all know she can lead her own book, we should consider implementation based on how DC actually uses her.
The only way she doesn't have those powers is if Earth-2 Kryptonians metabolize yellow sun radiation differently and we have no reason to think they do so, at least not in a diminishing way (they've only ever been stronger, not weaker).