When it comes to crimping electrical connections you really can't crimp with a pair of pliers. You need some sort of tool to apply TREMENDOUS pressure to the crimp area to get the copper wires and the connector to actually "flow" together. You can buy crimping tools at any Home Box or hardware store. If you notice, -the crimping jaws are very close to the fulcrum in the pliers where you can get that tremendous pressure.
I recommend crimping AND soldering the connection. If you can't solder then dip the wire in dielectric grease b4 you crimp the connection to keep moisture out of the joint.
Try to get connectors with weather boots and use wire that will just barely pass thru the boot in order to get a good seal. When you assemble the connector apply a little silicone dielectric grease to the wire so it seals that boot also.
Always use silicone dielectric on the connector tabs themselves for the final environmental seal.
The connector terminals in any of the connectors I have seen so far are very poor and do not provide a great amount of contact force which explains some of the problems people have with them. I don't know what company initially designed the connector, maybe Bosch, but if it was I expect better of them. Somebody should have made better ones but that requires an investment in tooling and R&D that nobody wants to do. These connectors were designed for OEM's that had no interest in making a connection that would last any longer than the warranty.