Completely subjective opinion that you may ignore: The wasteland. I know it's meant to make people uncomfortable, but yeah, traveling through it makes me want to strangle someone. And I'm still not sure why I couldn't find Squirtle even though I lit everything red and went on a rainy day to the spot where it's supposed to be, but it's not, and the thought of redoing it again makes me want to rip skulls in half. So, I'm going to let it drown in mud and die.
And puzzles can go to hell I'm using guides on ALL of them.
Subjective opinion that you probably should consider: Lots of side quests require you to backtrack an insane amount of times, and attempting to find Event Pokemon sometimes feel like whack-a-mole on the entire Reborn map.
Silent protagonists are staple in literally every official Pokemon game ever, but in this game the player feels so god damn powerless. Coming face to face with Corey and seeing him jump and the protagonist doing NOTHING - and the dialogue suggests he said NOTHING either - is... like wow. There are also a lot of bad moments that could've been prevented if the protagonist said or did something that's 'common sense'.
Also, I'm a new player and I haven't finished this yet, but are we truly expected to win EVERY battle, even 'impossible/you-can-lose-and-still-proceed' fights, to get the best ending? Not to mention the choices you pick such as Sirius or Sigmund battle. This seems really very unnecessarily harsh, considering that replaying the whole game *shudders* takes a minimum of tens of hours. Don't take this the wrong way - if Evoas was allowed to expand to the same scale as this it would be my top #1 Pokemon fan game, but since it's not this is currently the indisputable #1 Pokemon fan game in my list. Nothing even comes close to beating it (although, somewhere in the world, a new Pokemon fangame plots to dethrone Reborn once and for all...), but at the same time, jesus christ lord save me from having to play this massive game again from the start, with no deviations allowed in either choices or battles (WIN OR RIGHT OR NOTHING).