I am wondering which adjective could describe something is unexpectedly tough but pretty fair (I have "neutral" attitudes toward it).
I found the final exam was so ***. I really should have worked harder.
Edit: I also want to use this adjective formally, like:
Although the test is a fair game, I have never thought it could be so tough.
Edit: I am so sorry for “nitpicking”. I would use this word in the emails to the professor who made that test, so I want to be as neutral and polite as I can.