Hi Harry, I have a great deal of experience with self threading screws (Plastite and several other types) into thermoplastics and soft diecast metals, in industry. They work great in thermoplastics IF the boss walls are thick enough and IF the hole dia draft angle and the entry chamfer dia and angle are the right dimensions as recommended by the screw makers and IF the recommended drive torque is not exceeded. Use in soft diecast metals are more challenging due to risk of head breakoff during driving, but again if manufacturer's recommendations are followed for the hole, they usually work fine. Taptite screws (finer thread, tougher steel ) are better for diecast soft metal.
Trilobular self threading screws like Plastite are far better than Hi-Lo or other types of self threading screws in plastics. Drive stress is far lower than other types especially Plastite 45 (45 degree thread flank angle) and the trilobular screws have better resistance to breakoff during driving and resistance to backout under vibration after install.
Plastite screws do not work too well into thermoset plastics like Bakelite or other phenolics, including Tufnol , they tend to cause stress cracking. In Tufnol they can initiate delamination. I usually design for through fasteners in thermoset plastic parts, no threads in the plastic, and plastic only in compression. Less trouble.
Plastite screws do not work well for joining mild steel, stainless steel plates or sheets, or machined metal parts with drilled holes. (and are not intended for that, but people try it anyway, usually with bad results).
If you have any specific questions about this, post them and I'll do my best.