Confusion.
If groups of people use different systems it's necessary for each group to convert any measurements back to their own system before they can be understood.
So somebody who uses only the metric system would have trouble relating to a system that used rods, poles and perches, furlongs, pints, pounds and ounces ... And both groups would scratch their heads when faced with a system that measured things in cubits.
I'll add to DD's excellent answer. Confusion is the big problem. A few years ago a plane in Canada was filled with fuel using the metric system because that was what the worker was most familiar with. But all the gauges were in Imperial. The gauges said full. The worker checked. But the tank was only half full. The plane ran out of fuel and the pilot landed on an abandoned airstrip at Gimley, Ontario. Only people who knew the area from wartime air force activities even knew about the strip. Everyone survived. This near miss was a huge warning to everyone in aviation.
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