First of all, Equestria Girls' world is a mess. Characters which were of wildly varying ages in the show are shoved into the same school, beyond all rhyme or reason, fantastic happenings are merely accepted, apparently never even spreading beyond the school, and the school itself appears to have no structure whatsoever. The last of those, at least, can be corrected, and in fact was corrected by Rainbow Rocks' pacing, but the second promises to make future events just a little stupider, and the first is a deep-rooted issue with no hope of correction. These are issues created in the first Equestria Girls, and the latter in particular only serves the purpose of fan service. The thing is, though, this is a lazy approach that prevents the world from ever quite cohering. An effort could have been made to have these younger or characters appear more naturally instead of just shoehorning them into the setting with no actual thought put into it, but instead we're left with a shallow facsimile of Equestria that lacks even the most basic elements of planning. There's potential to salvage this setting still, but there's no getting around that one inherent flaw.
However, that's not the most controversial aspect of Equestria Girls' existence. The spinoff has faced criticism for its very premise, and unfortunately, those critics are not wrong. The high school setting is a cliche of the genre, and unfortunately setting the film inside said school adds very little to the movie except for an easy way for the main characters to connect. Perhaps given more creative space a better idea could have come about, but as the first Equestria Girls was mostly just a mildly amusing extended commercial, there was a rather limited space given. The big issue with this basis in cliche, though, is that the initial conceit of the series was to not give in to the pandering tropes of the genre. Thankfully neither of the movies dips too far into cliche, making valiant efforts to tell stories that keep with the spirit of the show.
At the same time, placing these characters in a different context does so very little to actually say anything new about them. The first movie is more or less a transplant of characters from the show into this high school setting with as little change as possible. That was actually reassuring after all the fears surrounding that movie, but that does decrease the spinoff series' reason to exist. Rainbow Rocks' introduction of Sunset Shimmer as a recurring element puts something interesting into the universe to make it somewhat different, which also allows the plot to diverge from mirroring the show's.
A bigger issue is Hasbro's obvious lack of confidence in the films. These got limited release, with Rainbow Rocks merely being a special event in my city, no previews or trailers beforehand. Of course, the movies' dependence on show canon does make them better suited to the TV screen, but it's still clear that Hasbro sees these as mere cash grabs, and as a result it's questionable how much freedom the company will give to future installments in the series. With an actual My Little Pony movie being announced, it's most likely that Hasbro's cinematic efforts will largely be focused on that, and thus the continuation of the now-intriguing Equestria Girls storyling would perhaps be best served as a television show.
Rainbow Rocks left us with an intriguing cliffhanger, and the Equestria Girls series has an interesting idea in how its big villains remain in the world after being defeated. The main show has begun to take that approach to an extent, but in Equestria Girls there's potential to do much more work with these villainous characters, as has been done with Sunset Shimmer. I think that the future of the series really does lie in another show, as that would better serve character development and reduce the wait time between instalments. Given the hype after Rainbow Rocks, that would be a good thing. Either, way, I'm intrigued.