A lot of the emotional moments the series tries to push don't have nearly as much weight as it thinks they do. And when they do finally managed to strike gold, it's usually ruined by an aggressively unfunny horribly well timed joke, or just it's future consequences and impact are left underdeveloped. For every scene in FT that got yo ass cryin' in the club, there is another scene with a shallow payoff that the show tries to paint as just as important.
The end of Natsu vs Zeref was truly great, Natsu's own journey of pain and suffering, a journey he never once let anyone see because he had to keep moving for the sake of everyone, clashed against Zeref's own pain. Natsu, with everything to protect, but never has the strength to protect it. Zeref, all the power in the world, with nothing to protect. Two brothers standing off, burning bridges instead of building them. Both threw all their emotion, their anger and sorrow and grief, at each other. Natsu won because Zeref didn't have anything to fight for, which wasn't his fault.
The dragon slayers vs Acnologia was a rushed and sloppy end that left Acnologia stale and unexplored, as the only true connection he has with anyone is Natsu, yet they don't play with that at all. After pulling the Fairy Sphere and Seven Dragon Fire out of nowhere, they flashback to Igneel's death, as if this is any sort of meaningful revenge. It's not, Natsu got thrown around, didn't get fired up, he promised his Dad he would get stronger and use that strength to squash Acnologia, he didn't. Yeah, Natsu being too weak on his own and having to rely on his friends is a series staple, but in a fight as deeply personal as this, what would normally be thematically consistent ended up throwing away one of the main themes of the Zeref fight. Which was that Natsu can stand on his own and be just as strong even with no friends to help, because fighting for them is what gives him power, not using their power itself.
I could write a thesis on this but it's pretty opinionated. As I pointed out when I explored how Natsu vs Gildarts and the lesson it tried to teach completely undermines Natsu's character, everybody wants different things from a story.
