Let me explain.

House of Hades had a lot more darker and more pressing elements to be a better finale than Blood of Olympus for the Heroes of Olympus series. I feel like if the two books swapped places, it would've been better. 

Everything that happened in Blood of Olympus was disappointing, but only because it was supposed to be a finale, if it was the fourth book, Gaia rising and Percy and Annabeth in Tartarus would've been in the fifth book making it a better finale. 

There would have to be some editing in the story, but everything that happened each book would've been better.

House of Hades would've been a good finale, Blood of Olympus isn't.