It's been discussed in this thread before, but Oda's made the female characters so weak and fanservicey it's almost unbearable. I hate everything about Nami, Robin, Rebecca, Shirahoshi, etc. since the time skip. I am crazy hesitant to have my daughter watch OP if she wants because of how women are portrayed. It was bad in spots (Nami/Kalifa in Enies Lobby, Nami at Thriller Bark) before, but it's terrible now.
I don't mind almost everything else mentioned in that Reddit thread or any of the complaints I normally hear. Just the females thing, man.
Nami was never meant to be a fighter, she's also the only person on the crew who actually knows how to run the ship. She's basically the captain without being the captain, and also what hurts her is that she never trained physically like most of the to her strawhats during the timeskip, she basically learned weather a lot better.
Robin is always deemed to be a "assassin" character and not want who's super good at dealing with opponents head on. I don't think she's much of a person who likes to fight either, just that she sometimes has too. I betcha she's kind of sick and tired of the whole "fighting for her life' constantly thing after growing up with it her entire life.
And hell, this also ignores how Chopper, Brooke and Sanji have been delegated to doing nothing since the timeskip. I think Oda is just at a crossroads where he doesnt want to have the "each strawhat crew has to fight the other equal member of the antagonist" during every fight. It makes the battles to predictable, and if there's one thing I love about One Piece is even though it's a formulaic shonen, it's incredibly non-predictable beyond "Luffy fighting major villain"
The fanservice stuff is bleh to me, but I kind of like their designs right now, they remind me a lot of the disney cartoon sexy women like a Jessica Rabbit, and One Piece reminds me a lot like a Bugs Bunny cartoon. I can see the complaints though, but I think that everyone on the straw hats has kind of become more fan servicey in just not always in sexual ways. (Chopper became a lot cuter, Zorro looks more badass, Sanji grew a sexy beard, Luffy doesnt button up his shirt anymore)
I'd imagine the real reason he does it though is because Oda probably love boobs.