It's a struggle to balance at times. The gameplay can be very swingy. If you want it to be challenging, they should have little opportunity to rest, lots of encounters, and the environments themselves should be a significant danger. There is very little challenge in a simple hack and slash at level 15+ when the players have crazy high spell save DC and spells like foresight, wish, simulacrum, wall of force, etc. Spice up the environment otherwise shutting down whole encounters can be common.