Going by the comments on this thread, it's hard to believe everyone's talking about the same book... (which I haven't read, and I'm now torn as to whether I should!)

It's doesn't go "off the rails" at all, it completely and satisfactorily explains everything. It goes fast but it's good. I highly recommend it. You won't want to miss the worldbuilding, it's absolutely incredible.

Yeah it's wild! Second-guessed myself a tiny bit after seeing all the conflicting views.

Minor spoilers:

I loved the world-building and mystery in the first half, the main character lives in a post-apoc monastary, tons of great details and tidbits. It felt vibrant, deep, reminded me of Redwall and the 5th Harry Potter, with the sexual repression and small-scale rebellion against authority.

He leaves the walls in the second half and stuff happens, I forgot most of it. The journey part felt like an afterthought. Something something aliens, really didn't feel earned or connected.