Why didn't Abaddon play more of a role?  He should have done something like fully manifest or possess the White King in the last moments so Dazen and he could have a massive boss battle.

Why didn't we learn more about black and white luxin?  We learned next to nothing for what White does or what its physical properties are.

Why was Orholam being a literal deus ex machina with a plane necessary?

Why was Dazen's identity reveal handwaved?

Why bother killing Kip to bring him back?  Why bother taking away his powers just to bring them back?

Why did Dazen get all of his powers back?  A scene with him using white luxin to heal himself or something would have gone a long way.

There's more that I can't think of right now but..

I just can't help but feel like Brent stared into the Abyss and blinked.  He didn't 'stick the landing' as it were.  He went back on what made this series great - the deadly consequences and reality of the world.  It ends up being a fairytale ending that compromises the brutality of what came before.  It's not all bad - but it just does not impress me at all.