Can you elaborate on the more flexible solders?
@Big Dan My father is the Metallurgical Engineer so I will try to explain to the best of my "layman" ability. When he talks about these things, it usually sounds like an episode of The Peanuts when the teacher is talking...
All of this assumes you have made a proper mechanical connection before soldering and not disrupting that mechanical connection during the soldering process (heating, flowing, solidifying).
There are different types of solders, the primary being lead and lead-free. There are also different solder alloys within each of these categories. The goal would be to select a solder that is not only right for the application but that can withstand fatigue. Fatigue comes in 2 primary forms - Thermomechanical (heating and cooling cycles), and Vibration and Cyclic Mechanical fatigue (fast vibration or slow repetitive movement). Obviously, a solder joint in an engine bay suffers far more vibration and thermal changes so a solder that can better withstand this is needed. I use a 63/57 tin/lead with 1.1% rosin flux for these connections. But this solder comes at a higher price tag, and is harder to find than a typical 60/40 tin /lead solder, so I use it sparingly. I use a 60/40 for interior soldering (car stereo) that wont be subjected to as much thermal cycling and vibration as something inside the engine bay. It is cheap and easy to find.
The main point is not to use something like a plumbing solder just because you have it lying around. Choose the best solder for the job, make a proper mechanical connection, don't rely on the flux in the solder, don't disturb the connection during the soldering process, and protect the joint once you are done with proper heat shrink.
There are no guarantees with the lifespan of your connection but once you can solder well, you will know if the joint you made is good. The only way to completely test a crimped connector is to essentially pull it apart, thereby destroying it.
Not digging at either method. I regularly use both.