Mismatched tires are a recipe for disaster, mixing radial with bias is the biggest no no, they are designed differently to deal with side wall flexing (keeping the tread on the ground, one gives the other does not
Mixed matched tire sizes really doesn't cause any frame problems since the suspension deals with that. The problem is weight carrying capability, just think what is you get a flat in a spot where you must drive quite a way to get to a safe spot to change, like in the mountains or a bridge, the weight that tire carried is now displaced to the other three.
Also ST tires have a little stronger sidewall but the main difference is in the materials used to make it, the rubber is produced to deal with the sun also the rubber is a little tougher. Many people use car tires in lieu of trailer tires provided the tire is rated to handle the weight.