I'll preface this by saying I didn't compare the movie to G4 at all, and I won't. No matter how perfect they could have made it, there was no way the movie would ever live up to the magic of G4 and all the memories we made with it. FiM was its own thing, and so is G5. Not to be cliché, but G5 the start of something new; thus, it will take time for it to develop its own magic, and for us to make memories with this new generation.
Anyway, I'm rambling. On to the movie.
I thought it was pretty good. If I'm gonna nitpick, the ending wasn't as climactic as it could have been. It just seemed a bit too sudden to me. That's something I noticed throughout the whole movie; a lot of scene changes and big emotional moments happened too fast. Even a short two-second addition could've helped blend everything together.
To use the ending as an example, after the lighthouse falls over, Sunny immediately seems like she knows the answer. If for a brief moment she looked lost in thought, or maybe an expression showing she'd suddenly figured it out, then it would have flowed better. But she's just stone faced the whole time. That kind of thing is a minor detail throughout the rest of the movie, but at such a pivotal moment, it's a little immersion-breaking.
Also, I don't like how they completely glossed over Sunny becoming an alicorn. I think it fits that she became one, but it barely gets acknowledged in any way. It's just kind of, "Ope, Sunny is an alicorn now. Let's wrap up and go home, nothing special here! We see alicorns all the time!" It doesn't taste good after a mildly weak climax (imho). Becoming an alicorn is a huge deal, and it should have been treated like one.
I do also wish we could have gotten a little more backstory on what happened to Equestria, how it seems almost all memory of the princesses got erased, why did the windigos not show up, etc., but I can understand why they weren't included in the movie. They've got plenty of time to explain all that in the future media they've got planned for the series.
But like I said, I'm nitpicking. It was a really cute movie overall, and I loved it (especially Izzy- her voice acting and character design are on point, and she's so funny). I can't wait to see where G5 goes.