It puts a floor on the player's skill. That's not the same as being easier. There are game states that are certainly winnable without hard mode rules but do not allow the player to force a win in hard mode.

My problem with hard mode is it leaves a lot up to luck. If it was 3 letters and ended in “at” it could be pat, mat, bat, hat, cat, rat, sat, etc. so all you can do is to pick one and hope for the best.