Necrozma's transformations were particularly underwhelming. Like, this mysterious parasite possessing the two highest Legendary Pokémon in Alola, one of whom is a previously established character whom our heroes are strongly attached to, should be an incredibly dramatic and impactful moment, but between the animation being minimal, the storyboarding making no attempt to play up the moment as a big deal, the soundtrack just being pretty standard battle music (at least in the Japanese version, I dunno what the dub did) and the sound-effects for it likewise being pretty stock...
It just comes off as "Well, that happened" rather than "OH MY GAWD THAT IS HORRIBLE WHAT ARE THEY GOING TO DO?!?!?!?!?!?"