To be fair, the problems you mention are caused by the confusion between the two systems, not by the use of the medieval system by itself. To avoid this type of confusion, airplanes everywhere still have their altitude measured in feet and speed measured in knots. This is true even for airplanes made by Airbus and Embraer.

However, the medieval system does have shortcomings even without the confusion caused by conversion. For instance, adding fractions is much more likely to cause error than adding decimal numbers. That's the reason why machinists in the US have abandoned fractions long ago, using exclusively decimal inches in their measurements.