I do not believe that is a true conformal coating used for high-rel (air, ship, space) applications but rather the typical coating used for consumer or high-rel gear not needing to meet more stringent ratings for harsh environments. But I am not sure, been far too long, do you know off-hand? The conformal coatings I had to deal with in the past were much more difficult to apply (and rework) and created a much harder, thicker coating.
For many years exemptions were allowed for lead-free solder for medical and high-rel military and related products. Those exemptions were due to expire a few years ago, but I have been out of that world for about a decade, so have no idea what happened or what is required now.
Whiskers (tin, for lead-free solder, but other metals like Zn also create whiskers) are a problem for anything, not just electronics, though might be more a problem for electronics since they can create leakage paths and shorts affecting performance (to say the least). Last I heard the fundamental mechanism was still being debated, but it happens under normal conditions and over time. They do not shoot out but grow slowly (usually) over time. There are other problems with lead-free solder, such as voids created and expanding over time that reduce current capacity and increase resistance, but whiskers and good adhesion are typically cited as problems for consumer products with which I have dealt. It takes good cleaning and higher temperatures for lead-free solder to adhere, and even a good joint can exhibits voids (as well as whiskers) over time.