John Barnard contributed a massive amount to Formula 1. He's easily up there with Colin Chapman, if not more significant, and other designers such as Gordon Murray. Barnard is responsible for a lot of the precision and eye for detail that we saw from the eighties right up until now. He's certainly one of those people who took things on to another level.
I don't think he was caught out by aerodynamics. His Ferraris of the 1989/1990 period had nice touches in that department, but the problem with aerodynamics is how dependant you are on your wind tunnel and other infrastructure for success.