I guess you could say that’s true for some values of ‘pretty well understood’. Check this page out:
AFAIK bikes are still designed mainly based on experience.
As for something slightly different, but with something in common, last time I checked headlights are also complicated enough that there is no push button solution for designing them.
Of course, that there exists no all-encompassing, exhaustive knowledge on a specific topic doesn’t stop people from writing treaties and textbooks, or design aids from existing.
Or YouTube videos from being made.
Also, the exact action mechanism of NSAIDs is still not known (this is also on Wikipedia); Aspirin must have been around for over 100 years at this point, and its natural predecessors for who knows how long.