Oh yeah, Luna's did happen at the snap of a finger and we didn't get to see her again until her Nightmare Moon episode (basically just her keeping up with the times) and a few other times. Everything else was a small scene or appearance along with maybe one or two dedicated episodes for her and Celestia.
Though concerning the negative feedback with Starlight, I think this has more to do with how the show writers introduced a new "main" character along with how she was redeemed.
Honestly they did shove her in the front over lots of other characters since the show writers deciding that Twilight should take her under her wing. That decision seemed to claw itself deep under people's skins. It seems to be a common thing when a show and the main characters at least are established, then suddenly you gotta tell viewers to focus on a new character and the compromise being to push one of those established characters in the background, people hate it. Tribalism might have a play in it.
I would say she took up "too much" screen time over the Mane Six, but then what about the CMC? Granted they didn't get a redemption arc (I think their purpose was to be more relatable for younger viewers while the Mane Six were the adult role models) but they did get a good chunk of episodes dedicated to following them while the Mane Six made a small appearance at most for those episodes. Yet I don't see much hate or criticism about that. Not to the extent as Starlight anyways. So I consider that a weak argument from the Starlight haters.
None of that is really a huge problem for me though.
But anyways, I guess another interesting example nobody's said yet is with Discord. I think he's a better and slightly more nuanced example of this over Luna, where basically mentioned above they just made her good for the rest of the show after she was defeated as Nightmare Moon in the beginning.
It was actually a very interesting choice to make him a reformed villain considering not just how powerful he is but also how much of a gaslighter and manipulator he is, even more interesting was making Fluttershy be the one to do it. He even was going to cause havoc but wanted to keep Fluttershy as a friend since she was trying to be inclusive for him, so he rolled back on whatever chaos he was causing. This was all while he was trying to take advantage of her. That happened all in one episode and his change in heart felt very sudden, yet a lot of people love Discord over Starlight regardless. Plus he makes multiple appearances later in the show.
Not to mention that he, for whatever reason, fell for Tirek's trick (or maybe it was intentional, but it seemed like he actually believed Tirek would make him powerful too).
And then later near the end of the show when, as Grogar, he was getting the rest of the villains to mingle (Sombra, Tirek, Cozy Glow, Chrysalis) so Twilight could defeat them all at once to make her feel better and more confident for what she and her friends were doing, but oops the villains used Grogar's bell on him and he lost his powers when he was intending to be a safety net if everything went a bit too far; his intention was good but it was just an awful idea to begin with.
And then he implied that there were more battles or even adventures they did leading up to that moment that might've been him causing them just to boost their confidence levels. It ended up doing the opposite when they learned about it.
Meanwhile, Luna and especially Celestia were about ready to turn him back to stone/imprison him right then and there!
On the other hand, which is the way that I see it, perhaps it's better punishment to just take away his powers because of the situation. After reforming, he wasn't trying to take over Equestria. He was just trying to set up scenarios for the Mane Six (especially Twilight) to help show Celestia that she was capable of being a great leader. Again, heart's in the right place but it's a terrible idea because of how insincere and manipulative it is.
I think it's a good example where someone who's redeemed themselves does something really stupid and in some people's eyes, they haven't changed at all. Though we know that Discord was not trying to put everyone in danger (I mean, literally everyone's life was on the line here), at least two characters were more than angry at him because that's what he ended up doing. Not to mention him still gaslighting and manipulating in the background all the way through until it was almost too late. It's understandable why they were angry at him and wanted to imprison him.
This is a bit of a tangent but we can compare that to how Celestia was letting the Mane Six (especially Twilight) "prove" themselves. I know some people consider her a bad teacher, but at least she isn't Discord levels of bad. Celestia observed from a distance and let them do their thing. However, if things get WAY out of pocket, she showed that she will step in (she did this during that episode when Twilight forgot about sending a friendship report and tried to fake a friendship problem to "solve", she stepped in to revert what Twilight did). She never made up scenarios for them to deal with to make them feel better about what they were doing like Discord was doing.