At AMF boats we always aim to build our boats heavy and robust. If an aluminium boat isn't built strong enough it will fail. If aluminium moves it eventually work hardens and cracks. But it is not all bad news, as well constructed aluminium boats have proven them selves to be around for decades so far.
When we lay the keel on our jigs we use a 10mm x 75mm extrusion, we then individually weld each 6mm or 8mm hull plate to this keel bar, then fully weld either side of the keel bar inside the hull, and a wear strip or doubler pad is fully welded to the hull. with six full length welds the full length of the hull this process creates an extremely strong and robust keel structure.
At worst I have seen boats with no keel bar and no wear strip and hull plates are literally butt welded together and ground off on the outside. These poorly built boats will not last!