I feel like I could write a full thesis and essay on the reasons I don't like This Bites, but honestly the two biggest reasons are character bloat as you mentioned and it going full blown reaction-fic post Alabasta. Cross gets the ability to broadcast to the entire world, and immediately every single chapter will now have random pointless asides of "look at these major power groups listening to the Cross Radio Show" from the Marine admirals to the Celestial Dragons to Shanks. I stopped reading it years ago, but no doubt if I went back now the newer chapters would have reactions from the Big Mom Pirates and Kaido's crew since they've both been revealed in the manga now.
As for the character bloat, it's mostly just that This Bites falls into one of the largest pitfalls I see all One Piece fanfics fall into - in the original work, there's a core crew of Strawhats that are just the right number of characters, maybe even a few too many since some like Robin and Brooke who joined later occasionally feel like they fall to the wayside. In fanfics? Every single author has favorite characters from every single arc who Oda left as one-offs that they feel the need to add, from Kaya to Gin to Vivi to Rebecca, and that's just more major characters who could conceivably fit in. Then This Bites has characters like, as you say the goddamn Kung-Fu Dugongs. And not just ONE of them; if it was only Boss it could probably be manageable, but they throw in four more as blatant Teenage Mutant Ninja Turtles expies and I just don't give a single shit about them as characters. If the story desperately needed turtle seals as knock-off TMNTs, they would have been best suited as a one-off gag or minor side characters who occasionally get brought up when the Dugong pirate crew is mentioned.
