Solder will fracture if the wires are not supported , ie wires moving at different speeds/angles. This is why mfg do not solder except on boards (the board provides support for the wire/contact that is soldered).
Ideally, you should use a proper guaged wire crimp (this is how the pigtails are constructed). If you want to solder, you can reduce risk by a) tying the wires together so they will vibrate together at same direction and speed and b) the them to some metal to reduce the general shaking by the wind.