So my question is, should we just solder all splices or how do they hold up?
Properly crimped connections do not need soldering. When I choose to solder them, it is usually because I did not have a proper crimp tool so the crimp I did was to make the soldering easier.
One area I choose to solder properly crimped connections at times, is when I want to seal up ring terminals for weather exposed situations. I heat shrink the terminal/wire and the soldering seals the end up, prevent moisture and corrosion creeping up the wire. Terminals on boats or antenna rotator on tower tops for example. The hazard to be aware of is excessive wicking of solder up the wire flex zone. The larger terminals can be found with closed ends.