Any cpu/gpu cooler is only as effective as what the chassis allows.
The more obstacles present for airflow, and the higher their 'difficulty', the less effective cooling is.
An AIO is not a miracle fix for cooling - they still need air. They also take longer to cool down because of their higher thermal capacity.
If the radiator were to be mounted at the front, of which there are some pretty solid obstacles:
-Airflow loses more strength when it has to turn, as opposed to straight in and out.
-Those little side grilles restrict airflow. I've seen worse side vents designs, like on the Gigabyte C200.
-Fans draw air in a conical shape in front of them, but there's a wall in the way obstructing this and forcing them to draw from the sides. This goes back to point 1.
-The radiator itself also restricts airflow.
^The fans will have to work harder to get any reasonable air though, and fans having to work harder means more noise.
If mounted at the top, the grille and filter are the main obstacles. Other than that, it's a straight path up and out.
You can get away with running the fans at lower speeds here too.