I have driven many times from Cancun south to Puerta Aventuras and Tulum. Never had any problems with police or traffic.
One thing I would suggest is if there is any existing damage to your rental, even though they document it on the rental agreement, take pictures with your phone with the representative in the picture. They tried to stick me with $700 damage that was already there one time and I had a photo with the guy pointing at it. Nice try though.