Firstly Zeref is super powerful. It take a lot before Natsu is able to stand toe to toe. That battle shows that Natsu was about even with Zeref. But then Zeref gets the fairy heart a limitless source of magic. Zeref is then much more powerful than Natsu. Natsu beats him. After exhausting a bunch of his energy because he burns his feelings....what the hell does that even mean?
Hiro Mashima was like here ya go. If you don't like it screw you. Dragon slayers rule. Like I get battle shounen the power of friendship, there's an extent where that works it awesome I get. And then there's what happen in that fight. Natsu has exhausted his magic and Zeref has limitless magic now. He was already freakin ridiculously strong. Natsu power of friendship beats him as if the fact that it's limitless magic means absolutely nothing.
Fairy Heart could power the faces forever. Natsu's friendship power is greater than that. Natsu's friendship power is greater than infinity fairy nukes. It feels like he wrote himself into a wall and then said good guys win power of friendship.
And then Acnalogia doubles down on that. Acnalogia could devour everything. Well super power of friends himself plus the power of dragon slayers equals whatever the hell I want it to.
Like the fairy heart could have rejected Zeref considering who it came from and Zeref's whole motif that would have made sense even more so if it powered up Natsu since he's fighting for fairy tail and embodies its heart in that moment. But nah.
Acanalogia was beefed up to be too strong same issue with Zeref. This has to do with power scale. In fact I watched a video which explained the issue with constant increases of power scale for the next enemy because eventually you get I can make universes disappear by blinking and then you get some who's like I never blink so I win.