Solder joint reliability withstanding strain or vibration is all about grain size. Eutectic solder gives a smaller and more uniform grain during solidification. There are always other things in the solder, even when not deliberately adding silver. So the intermediate plastic phase allows for grains of other alloys to grow. Where with eutectic they tend to be frozen in place.
Lead free doesn't really have any true eutectic alloys so it's always a compromise.
The fatigue failure is a bit different between leaded and lead free solder. Lead based solder creeps. Grains enlarge, micro fractures develop at the boundaries and get larger as the grain size grows. Eventually a fracture will propagate across the micro fractures and you have a failure. Lead free solder is stronger initially and doesn't creep. Which means that grain growth comes from stress and fractures develop more quickly when they happen. You can look at a leaded joint and see a rough almost blistered appearance. It's starting to develop fractures and will go soon. With lead free it's hard to tell from the outside until a fracture appears, in which case it's often too late.
Disclaimer, back in the '80s I did a bunch of reliability testing on the then new SMT technology at a defense contractor I worked for. I was part of writing the book on how to make this work. I was the first to present to the DoD a successful run of 1000 cycles of thermal cycling from -54 to 125C on an LCC44, which was kind of a benchmark at the time. The pictures of LCC joints in Mil-Std-2000 and the copies in the current commercial industry standard IPC610 came from me and that research.
Thanks Stephen! Fascinating that the toxic stuff does in fact always work better!
One of the ancient Chinese secrets I picked up from the Non-chlorinated brakleen will do a hell of a job cleaning not just car parts, but tape heads too, and if used sparingly even resurfacing rubber rollers. It also contains substances known to the state of California to cause cancer and birth defects. Good thing I don't live in California!