Vaccines are like cars. They are designed with the best available technology and you buy them even though you have no idea how the hell they work.
Except for the workings of a car are well understood by the engineers who build them, while the workings of the human body and very complex biological instructions are not well understood at all.
Unforeseen complications are the norm in drug or vaccine development. This is why they literally use Guinea pigs to test this stuff.
Comparing what these people do to what car engineers do is very misleading.