I, too, have an '05 Moomba. I do not have the stress cracks that you are describing but over on the Moomba forum ( http://moomba.com/msgboard/index.php
) people have mentioned on some of the older models that when the swim platform was put on at the factory the installer missed the backing plates that the screws were supposed to screw into leaving the platform installed to only the fiberglass. I would take it up with your dealer as if that is the case with your boat it should be covered as a manufacturer defect.
I would not judge the entire SC line or Moomba specifically based on this. EVERY manufacturer has an issue now and then. Moomba is a price point boat but it is a good quality brand and Skiers Choice stands behind their product. Supra is on up there with all of the other "Luxury" brand and priced boats and yes, there is a difference in material quality between the Supra line and Moomba line.