That's a good point, that they can't really let loose with their powers outside of controlled conditions. Maybe they could in outer space? (Although we haven't seen if they have something like transsuits yet.)
Sun Boy & Cos are shown to have a good understanding of their powers, or at least how to use them effectively. According to Brainy, that's not the case with Light Lass and Star Boy, who, he alleges, have extremely potent and untapped/underexploited abilities. Star Boy's been portrayed as a bit of a doofus so far, but I wonder if Ayla doesn't have a lot better mastery of her power than she's let on.
You'd think these four could easily have overcome those evil landscapers, Terror Firma, but perhaps they are more cautious of the negative, destructive consequences of using their powers and therefore always holding back.