Comment by sowbug

The usability issue with IUs is that people are used to scales measuring weight and containers measuring volume, but an IU is different for each substance.

Another issue is insulin syringes are labeled in "insulin units," which hapless folks reasonably assume can be abbreviated "IU."

If you are measuring out a certain number of IUs, and your calculator or formula hasn't asked you which substance you're working with, you're gonna have a bad time.