But that’s almost never how it’s written. In fact, Starlight has been easily beaten off-screen by villains TWICE. XD
And Pharynx kinda one-upped her with that sack, she failed to do anything meaningful to a Maulwurf, and Discord being affected by her magic was just for show on his part; he effortlessly circumvented it, and he was smug the very next time she talked to him.
She seemed scared shitless of Celestia and Luna after the fact as well, after resorting to it in panic, had to practically appeal to their egos, easily read as her fearing their retaliation, and it’d already been established in her debut episode that her power can affect alicorns, which does not seem to be because she’s stronger than them like losing offscreen to Dark Archmage King Sombra.
She’s never gotten locked down into a beam struggle with an alicorn and won, at least not without backup (a brief instance of Cozy Glow vs her being on the losing side being a noticeable example of that), and it also never comes down to her getting through their shields. I strongly suspect she’s able to be defeated by some named unicorns, including the guy who can cover an entire city (Shining Armor).
Starlight easily being able to do magic feeds into one of her big flaws, which is impulsiveness. She often does things on a whim, and since she has the power to easily act on those whims, it reinforces it.
She’s a pony that has less restraint than Twilight does; she might be more skilled at combat, but she’s more prone to letting her emotions get the better of her judgement, whether it involves magic or not, because she didn’t have a mentor or the life experience to teach her discipline in that way, for a long time.