The main issue with the aluminum conductor is that aluminum expands more than copper when heated. When aluminum wiring warms up, it expands more than copper does, and when it cools down, it contracts more than copper does. This expansion and contraction, over time, will allow for loosening at the connections which can lead to arcing at the point of connection.
To make the problem worse, all metals oxidize or corrode in an oxygen environment. Copper oxidation forms as a conductor and when it comes to electricity, a conductor is a good thing. However, aluminum develops as a resistor. This resistance causes heat. Oxidation accelerates when two unlike metals are in contact with each other. This may be part of the source of increased resistance when aluminum wire joins to outlets or switches intended for copper. Eventually the wire may start getting very hot, melt the insulation or fixture it’s attached to, and possibly even cause a fire.
This photo shows what can and will happen when aluminum wiring is connected to a receptacle that is marked "USE COPPER WIRE ONLY FOR ALL CONNECTIONS". This example also shows what is stamped on every receptacle and switch. I'd also like to mention that this particular one was smartly done with easily read markings (even though the warning was ignored by the installer). But beware, not all receptacles are marked as clearly. The information is there but most are stamped so tiny that it's difficult to read.
Again, the problem is with the connections. Aluminum wiring in itself is not dangerous. Aluminum wiring, when properly installed, can be just as safe as copper. But if it has not been installed properly, the connection —where the wires join to the outlets and switches—can present a fire hazard.
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