I guess the problem with most solder joints with bigger wire is not getting the solder to the middle of the wire, so crimping is usually preferred. That said i solder most of my stuff, but am crimping curious
. I've soldered battery connections and welding ones and never had a problem myself, and the solder really locks on tight for welding lead and wont work loose in my experience, that's what i have on my stinger i use, but the tweco ends i use on the lead have a big allen screw to act as a mechanical joint. Solder is also easier to do without a crimp tool (which i don't own), so that's what I've used since i have the stuff needed on the welding rig or at work. I'll brush the lug or rough it with a file, emery the wire, fill them completely full with flux, and use a torch gently on the lug while it's held vertically like a cup so you basically pour it in. I also try to really take my time so the heat has time to work into the wire, heating and dabbing more flux for awhile until i think it's ready.
I've used plumbing solder and flux since that's what i have on hand, and I'll fill the entire assembly with solder. A wet rag will help with not melting the insulation too much, and ideally when you're done you can soak it in water and stir it and rub it a bit to remove the flux. Another cool trick for cleaning the copper before hand is to flux it and heat it, and then wipe it off with a rag, doing that a few times will really shine it up and remove the oxide layer. And of course tinning the wire beforehand is a really good idea too, so I've been doing that more lately which makes it much easier. When tinning the wire I'll fan it open so i get it all, and then gently heat and hammer the wire in a tight configuration so it'll fit in the lug. While I'm certain this isn't the correct way it's worked for me and it's cheap, but I'm pretty comfortable working with solder and a torch