Aluminum by nature is a softer metal that copper, it contracts and expands more when heated and is more susceptible to oxidation, which is a thin film on it's surface caused by oxygen. It should be noted that this pertains to only to the smaller single wires that you see on wall receptacles and switches. The aluminum stranded wiring you see on the 220 volt appliance and air conditioning wiring and circuits do not pose a problem. When aluminum wires are only twisted together, like you would with copper, the wires expand and contract then current flows through them which produces heat from the poor connection and if the wires has oxidized, this will heat up the connection even further, causing a potential for overheating and a possible fire.