It seems to me that all those who claim to ‘use the metric system’ don’t. If they did airspeed would be in m/s, fuel flow in kg/s, altitude in m etc. At least pressure is now in hectopascals (in the UK at least) so we are slowly getting there!
The messiest aircraft I’ve flown for units was a PA32 with fuel flow in US gal/hr. Fuel loaded in litres. W&B calculated in lbs. Lots of opportunities to get it wrong,