Its true that Al is about 1/3 the strength of steel. It is also about 1/3 the density. Bike designers use more volume of Al, using thicker-walleed tubing. They also use fatter diameter tubing to build a stiff structure. Al will fail if you keep bending it, so stiff structures are used to prevent fatigue.
In practice a good Al frame will last as well as any good frame. Most failures are due to faults in manufacturing, leaving stress-raisers which concentrate stress, or giving heat-treatment during welding/brazing.
The one big weakness of Al is at the derailleur hanger. These do get mangled up more easily than steel, and cannot be bent back in shape. Even replaceable hangers have their drawbacks; the attatchent point can get bent, and you have to buy the replacement part many years after buying the frame. Will you find one that fits?