Biggest problem is cracking. The solder does not do well in vibration environments and a car is a vibration environment.
x2. The wiring will typically crack where the solder ends, we have seen it over and over on durability routes. From what I understand, connections aren't supposed to be soldered on airplanes for the same reason.
I must take longer than 30 years because my crimped and soldered connections that I made are still good. I think as long as you keep your wires from vibrations and run it in insulated clamps like I do there won't be an issue. I use crimp terminals for interior wiring or connections that aren't exposed to the weather. After 40 years as an Import technician I have seen my share of corroded crimped connectors here in the rust belt.
Gus
I started working for a small telephone company in 1964, the cable splicers would skin and twist the smaller gauge wires and insulate them with small paper or plastic sleeves depending on what the cables were insulated with originally. They used insulators over the 20 GA or smaller cable wires when splicing the telephone cables together, they would twist and solder the 19 GA cables and slide the insulators over them. It didn't matter if the cables were on poles or in conduits underground, they used the same techniques on both
I don't have a clue what they do now in this age of smart phones
