Guessed by @UnderwaterFunktown
Prime 3 is... a weird one. Whereas the first two games in the Prime series sticked mostly faithfully to the traditional Metroid formula when translating it into 3D, this game took it in a very different direction, to the point where you could almost say it's not a metroidvania anymore. A lot of the changes were exciting and done well. Lots of planets to explore? Awesome. Even more awesome when all of them are so rich with lore and breathtaking visuals, something Retro Studios really excels at. Being able to go inside Samus' ship was also a first for the series and just overall really cool.
But many of these things brought their problems with it too. The game tries to go for an "epic cinematic" story, and sadly it falls flat because the writing, voice acting, and cutscene animations are all really weak. I mean, a 2007 Wii game can only be so cinematic, maybe someone should've told Retro Studios that beforehand. I suppose they wanted to round out the trilogy in a bombastic way, instead of just focusing on Prime 3 as a standalone game, which is where it works best.
Besides, it's no longer even a trilogy, eh? I wonder if this will still be the latest game in the Prime series on my list by this time next year.
(Well... if there even is a list next year. Anyways.)