I don't know if this is a good place to do it, but I'm going to wager a guess as to one of the major reasons why this arc and modern OP are disliked. Imagine a little icon of Luffy running with a line behind him, like a loading screen in a video game. Pretend that the icon represents how far Luffy and the Straw Hats as a whole can go with their starting strength. At the start of the series, Luffy ran through East Blue without stopping (Unless you count Mihawk, but he wasn't really an antagonist). There were definitely some moments of danger, but in straight-up fights the SHs took the cake every time. Then Luffy runs into a brick wall during the Alabasta saga. The crew struggles more then they ever did before, with Zoro and Sanji getting beaten down hard and just barely scrapping out wins while the previously undefeated Luffy gets handed two losses in a row. It took everything the SHs had to break through that wall and was a great way to show that the Grand Line and the fighters in it were serious business. Later arcs like Water 7 and Thriller Bark continued to put up walls preventing Luffy icon from just running through them, leading up to Sabaody, which was an wall that wasn't broken. Looking at the time skip like a second starting point for the series, Luffy icon just ran straight through FI, PH, and most of Dressrosa. With Zou being a breather arc, this arc seems like the one where the first sturdy wall that's difficult to break through in the New World should be. And in terms of raw power, the Big Mom pirates are indeed a wall large and stronger than anything seen before. However, due the situation where only half the crew is present, this arc hasn't been as fighting-focused as most and the actions present instead can feel like convenient holes that let Luffy icon go under the wall rather than finding a way over it or partially breaking it. Thus, the wall looks really impressive, but is actually easier to get through than other walls that came before it. Of course, the SHs are better prepared after their training than they were for most pre TS arcs, but with how much the Emperors have been hyped, it makes sense and is reasonable that some people would want to see the crew struggle more than than have ever before. The separation of the crew itself is a also a problem, as now they don't succeed or struggle as one unit as much as they used to, so the single icon thing doesn't work as well. Sanji's icon would repeated run into walls without eventually breaking them (and this arc hasn't changed that yet) and Zoro's icon would not even have come across an anthill yet, even if he is running in the wrong direction. Everyone else is somewhere in between them, and with less crew interactions the SHs feel less connected than they have in earlier arcs.
So, um, that's all, and if I'm horribly off the mark (or just suck at writing XP) please tell me so that I may be better informed about what people currently think about the series (or me writing).