When I build a new harness, it's solder and heat shrink. Even with crimp lugs, I will use uninsulated lugs, crimp, solder and heat shrink. This is the cleanest and least bulky way IMO. Solder properly, don't over heat the joint and don't over fill it.
I also use 3M crimp connectors, but I don't prefer them. They have their place if you don't have soldering skills, are doing some retro work that is already crimped, or are in a get-it-done situation (side of the road). There are lots of situations where factory installs use 3M crimp connectors. Examples include truck bodies, marine, electrical switch gear. The key is to use really good crimpers.
Scotch Locks have a place. They are not my go to. But, for emergency repairs they are great. Or or something you know you will redo later anyway. I'll give another vote for tapping into factory vehicle wiring for trailer lights. If it's my own vehicle, I will cut and solder. But if I'm helping a buddy and he bought a kit that has Scotch Locks and that's what he wants, that's what he gets. Saves me time and my own supplies.
Years back when I was a mechanic, I had a customer come in with a horse wagon they had just purchased. Not a trailer, a wagon they could tow with a vehicle and also be pulled by horses in a show type setting. The tongue wiring and remote battery wiring were a mess full of crimps and Scotch Locks. Most of the lights didn't work. I expected all of the wiring to be bad. And, at the same time I didn't think the customer would accept paying for it. I got under the wagon and discovered the wiring was the flat four wire ribbon. At each side marker location it had a little curled loop in it and the appropriate green or yellow wire was peeled of the ribbon and a Scotch Lock connected it to the side marker. White ground also peeled and looped. Very clean and nicely done. Once the tongue wiring was fixed back to these runs of ribbon cable everything worked fine. I was impressed with the workmanship and it tought me Scotch Locks have their place.
