The yield strength of most alloys will reduce about 40% in the immediate vicinity of a weld (up to about 2-3 times the material thickness).
It's also harder to verify the integrity of a weld compared to a rivet.
In the automotive industry self-piercing rivets and epoxy seam adhesive is common practice in aluminium bodies, but the SPR technology is better suited to high volume production.
Rivets aren't as strong as the surrounding material, but their performance is consistent and they don't create a heat affected zone.
Pressure and fuel sealing is achieved by 'wet rivetting' with a poly-sulphide or epoxy application to the mating surfaces, which has the added bonus of creating a stiffer structure.