Soldered joints on solid copper wire are ok, but you still need a wire nut or other connector, since soldering is not considered a "mechanical connection" in the code.
Some of the original wiring in my house from 1929 is with cloth insulated BX cable, where the wires were twisted, soldered, then held together with ceramic wire nuts, and then taped (with cloth tape). It's perfectly fine, although I've been replacing it as I encounter it.
Soldering stranded wires which should be crimped (when they're subject to vibration or stress) is a no-no. It wicks up between the strands, which stiffens the wire, which puts more stress on the joint. Left unsoldered, a crimp allows for more flexibility, and properly crimped, the joint will be too tight to allow for oxidation.
As for whether a "plastic" screw on connection can handle as much heat as solder:
I have seen all plastic wire nuts with plastic threads inside. I NEVER use them. Total garbage. Real ones have a special metal spring inside. As you tighten it, the sharp "threads" facing inward bite into the wires, holding it tightly in place, and the spring maintains compression on the connection.
