Sooooooo many things to say about this movie, but they're not in order:
1) So that thing about Ash and Volcanion butting heads was kind of a lie. While the poster showed them glaring daggers at each other with those rings, they never really got that far. In fact, they only started bickering the moment the rings came off...and even then, it lasted like 5 minutes before the climax came in and any ill-hatred Volcanion had was covered because everyone was too busy fighting the villains.
2) Safeguard doesn't protect you from multiple Hyper Beams! Talk about hax.
3) And speaking of hax, that Alakazam had Mewtwo levels of psychic power. Effortlessly lifting more than 4 people...and one of them was a freaking Steelix. Even without Mega Evolving, it was a serious threat...that never came to be.
4) Figures Clemont's first crush would be a mechanical Pokemon.
5) Racel is the dumbest, most oblivious character I've ever met. Forget about the fact the obviously-evil villain had something to relate with you. How the hell do you ignore the fact he openly is seen torturing humans and Pokemon in a peaceful habitat and say, "Hey, sis! It's all good! Jarvis says this is for the good of the kingdom!" In fact, the whole plot happened because of his massive idiocy. I would have let it slide if he had said anything when the climax was starting, but he's still smiling and being happy after Ash and everyone else got tortured real bad?
6) So Kimia mentioned their father...but he doesn't appear? Even when the fortress is leveled?
7) Exactly what caused the city to get wrecked in the past that forced Eliphas to hide Magearna? Had Jarvis have some sort of connection that related to the past, he would've been a better villain.
8) And speaking of which, it's so hilarious they tried to really, really make him evil for the sake of it. "I got my weapon now. If I want to fully control it, let me destroy its home!" That only took the attention away from the city of machines. In fact, the city becomes forgotten and once more we're back to the wilderness. The previous movie did have a city, but it was so flat and casual anyhow that it wasn't remarkably memorable. Back to Jarvis, he should have been a descendant of whatever caused Magearna to go into hiding hundreds of years ago.
9) Can we get a clear explanation how those Mega Waves came to be. Also, way to say how heinous it is to disregard the bond part.
10) There's something wrong when even a full army of Mega Evolutions can't even hold Serena or Clemont back. Either Chespin is Brock's Happiny levels of strength, or they were just filler.
11) Despite the fact Ash-Greninja gets its own score, he doesn't do anything remarkable other than luckily derailing Jarvis' escape for a little bit. Essentially, he gets downgraded in the action department but is lucky to be in the camera's line of sight. Pikachu instead takes more action than he does. But hey! Ninja frog gets his own portrait in the end as if he did something!
12) Holy crap, did Volcanion incinerate those Heracross and Pinsir?
13) Where did those other brainwashed Pokemon go? Especially that Shiny Gengar?
14) Well, Swampert wasn't properly shown in the anime...so may as well brainwash it to be evil. In fact, none of those Mega Evolutions haven't been seen in the anime... It's like they wanted to get rid of them before the generation ended.
15) Nobody bathes an eye at Z-1 transforming but Bonnie is only proud for a moment without questioning it happening. Even when Perfect Zygarde showed up, Kimia just said the Pokemon of the plains stopped the beam. What?
16) Keep being a jobber, Noivern, or at least the target of machines coming out from the ground.
17) Magearna was actually endearing and has more personality that Clembot, but just for a little bit.
18) That ending was too quick on the emotions. Volcanion is dead! No, he's alive! No wait, he's dying! No wait, smell these flowers! That ruined the mood.
19) As always, humans are all horrible because of a few people Volcanion had a beef with. Specifically, everyone is a monster, even though the first few he met didn't exactly do anything bad the moment they met.
20) I liked to see Ash being treated as a human yo-yo with amusing injuries.
21) Did Disney already infect the dub with loud music? I could barely hear the characters talk.
As a whole, this was the best XY movie, but only by a little, and it still doesn't beat some of the past like Lucario's or Arceus's movies. Hell, the DP movie era was pretty good, except for maybe the Shaymin movie.