The points of failure in any portable computer tend to be the things that move or get plugged/unplugged - especially the power jack. My son had a Compaq laptop that was just fine - except he had abused the power connector so much that it failed, and it was soldered to the mainboard, rendering the whole laptop junk. At least on the Minis, the power jack is on a small, replaceable board, but if you take care not to put strain on the connector, it should be good for a long time.
Components, such as capacitors, do fail, but you can't predict that. The parts that do tend to have known failure modes are easily replaceable (hard drive, battery), so that's ok.