Having been an electronics tech in the Navy and trained in proper connection making I have some input. I use uninsulated connectors. I crimp them, I solder them and I use heat shrink. As I was taught the crimp is for the mechanical connection, the solder is for the electrical connection and the heat shrink provides both insulation as needed and strain relief.
Now you will hear people say you don't need solder for several reasons. Such as it's not needed to prevent corrosion, well that's not why it's used. Or that it wicks up the wire and makes it stiff and subject too breakage. In this case there are two issues, too much solder was used if it wicks up the wire and improper strain relief was used, if any was used.
When you solder you want to use just enough to electrically bond the pieces together. You want to see the strands of the wire and a nice fillet between the wire and connector as applicable. Such as below.
Now to strain relief, most of those telling us to not use solder are electricians. Now electricians are general decent folks they are not experienced in dealing with mobile environments such as a warship, or a vehicle. Where vibrations are plentiful and climatic conditions can vary widely. You use heat shrink as a form of strain relief, OEM's don't but then their connectors have built in strain relief these days. You should also use harness tape to tape the wires together, it not only looks neat and professional but it is also a form of strain relief and use the harness holding clips or add them if you have none.
Now some might point to how the OEM's do their wiring. Which can be a good guide if you understand why they do what they do. They don't generally solder connections in the wiring harness. True, they use crimps because it is cheaper and faster than crimp and solder AND it only has to last till the warranty expires. As for a crimp being corrosion proof,or gas tight. Not in a mobile application. Look at the OEM's connectors these days, they have all sorts of seals and gaskets built in to protect the connection. See below
Now you can get fancy and use Weather Pack connections if you want, I just use what I mentioned with a bit of dielectric grease if necessary. If you want to use those fancy connectors NAPA has a decent online tutorial.
NAPA tutorial
To close, whenever I see this in a vehicle I shake my head.
The problems with this connector are manifold. First the plastic part contributes little beyond insulating and even that can be compromised by an over crimp. Next, even if you believe in the crimp only theory you cannot do a visual of the connection because that plastic bit hides it. They provide no strain relief either. The sole benefit of these are cheap and easy. Not good though.