Hidden achievements are fine. They let you compare with other people without spoiling secrets. You can't see them without getting them, which has some issues, but it's alright. I'm sure there's a way to view these anyway. The riddles basically fall into this too, except hiding them with a trick rather than a hidden tag.

Challenges are great. They don't really show progression, but it gives players something to do if they want. I don't think they add anything personally, but I'm fine with them.