I'd say the biggest thing about the day/night cycle is not necessarily how difficult it is or isn't to implement it, but rather if it's worth it compared to what it would add to the game. If it's just a graphical effect it's probably relatively easy to implement, but also doesn't really add that much that can't be gotten with just static day/night settings. If it's more than that it would take more work, but would it add enough value to the game that it would be worth spending that time?