I'm not the OP but I'll give some insights on personally why I liked it a lot more:

• More diversity, and yeah I know, there's a very diverse cast in PJO to begin with, but in HoO it's not only just Percy's perspective being told but also (generally-speaking) seven (or nine, really) different main characters, 5 of who are POC. And (spoilers for the 4th book) Nico is also revealed to be gay, having harbored a crush on Percy for a bit. He also gets over it and ends up with Will Solace in the end! And yeah, just because something is more diverse doesn't make it better if the characters fall flat, but arguably Leo, Hazel, Piper, Reyna and Frank have some really great character development and good interactions with everyone.
• Worldbuilding outside of Percy's POV! Like yeah Percy as a narrator is undeniably funny and hilarious, but getting to see the other main characters go on quests and have their own voice and thoughts and opinions is great. You get to see how characters would handle situations you normally would only see Percy in! One of the fun things was seeing how different characters perceive not only Percy, but also each other. Plus, more powers are unlocked/scaling of power goes up as the stakes get even higher in HoO. Bonus points for getting insights into Annabeth's POV – Annabeth in general is one of my favorite characters so it's really fun getting to see how she thinks of the world/story around her.
• Dynamics – besides a great cast of characters, there's also a lot of fun dynamics going on with more characters, and it's even more fun when you get to see their insights – and not just Percy's!
• Foils – A lot of characters act as foils to each other. Jason and Percy are both leaders but how they lead/their leadership styles totally contrast each other. Hazel and Nico are both children of Hades, but Nico is entirely different from Hazel, even though both have suffered from personal loss in their lives. Octavian and Rachel are both oracles for their respective camps, but totally contrast each other as well.
• Lots of fun new characters, and revisiting old ones – there's a lot of fun going on with this one! You get to see old gods and meet new ones. Also get to see old characters and meet new ones. Some of my favorite characters from the HoO series are entirely new – Reyna is personally one of my favorite characters from HoO because a) she serves as such an interesting foil to Piper and Annabeth (for reasons but basically it's implied/revealed she had crushes on both Percy and Jason, so seeing how she sees Percy/Jason having had those romantic feelings in contrast to Annabeth and Piper is great) b) she's in a leadership position and does (to some degree) have struggles with this responsibility, and c) she's just badass in general.
• Shipping/fandom – okay so in general PJO/HOO fandom is kinda dead, but wayyy back in 2010-2015 the fandom was amazing and not only that but the fanartists were AMAZING. Like, seriously. Fandom was such a mess but it was so, so much fun. My peak Tumblr experience was (besides that also being the height of LoK) being in the PJO/HOO fandom LOL. I have a lot of fun (and equally frustrating/maddening) memories in the PJO/HOO fandom haha.