Any second-hand bike needs a full inspection. Whether there's likely to be a problem has more to do with how the bike was treated than the materials of the frame. A cruiser or hybrid in good cosmetic condition? I'd still inspect it, but not be too worried about it. There are aluminum bikes in my family fleet that have simply never seen hard miles or off road conditions. The only breakage I've ever experienced was a steel frame -- one of the fork ends broke off.