Recent mains ones are unlikely to be "transformers" - those have been pretty much universally replaced by switch-mode power supplies.
Some advantages - most SMPS don't care whether you plug them into 110 volts or 230; they are pretty efficient, and usually don't get as hot as a transformer when on heavy load. Disadvantages are that if they let go, they will probably take the device being charged with them (byebye iPad), and the cheaper ones cause interference on radio/television/WiFi.