Beached, Iíve been to AC 5 times in the last two years by myself, a friend and my wife. We have never experienced any safety problems. That also includes night bar hops listening to music. Of course I am careful and don't get crazy drunk, unless I have two designated "walkers" with me.
If you are in town or south of town there are always taxis if you have any concerns at night.
I've been to Isla Mujeres twice and IMO it doesn't even come close to AC. Great place and great people.