A properly crimped connection should have no room for solder in the connector. Most of the crimp tools I have seen on the tool trucks leave a lot to be desired.

Soldering and heat shrink tubing is fine for splicing 2 wires together. The trick is to use a big enough soldering iron or gun that can get the job done quickly before any solder can wick back up the cable and turn your nice flexible wire into a stiff brittle mess just waiting for an opportunity to break under vibration.