The hard ones are insane. Played through the whole game again to upgrade my legions to maximum and get better to finish those and it was worth it.
Honestly, after my first playthrough on casual even the first batch already felt insane, I gave up on them pretty quickly. Thankfully there's no story content locked behind any of it, not even flavor text or anything, so it's not like you're missing out if you don't feel like going for it. After my second playthrough the gameplay really started to flow and I got through all of the post-game, they can be some pretty satisfying challenges to complete.
Also, personally I think File 6 is the high point of the game. That chapter just adds so much to the worldbuilding and makes this game so much more than it otherwise is. Sector V shows a completely different side of the game's world, which otherwise feels pretty generic, and Kyle steals the show. The chapter's pretty long and drawn out, yeah, but I feel it remains interesting throughout and is paced amazingly well in how it builds up to its insane ending.
I'm not sure I'd say it's my favorite chapter, because the last one is breathtaking, but I do feel like it's the one that adds the most to the game. As in, if you were to remove any chapter from Astral Chain, File 6 is the one where it'd impact the game most negatively.