What are the major differences between the customary and the metrics system?

The metric system is easy to work with, you can do math easily accurately without a calculator because the conversions are all by multiples of tens. 10, 100, 1000, etc.

Celsius temperature scale is set at 0° freezing of water and 100° boiling.

The customary units in the US or even some in the UK are obscure number conversions, more error in calculations and temperature in Fahrenheit scale is arbitrarily set by the first reliable commercially available thermometer sold in the US.