Basically every statement that says "Never do A" or "Always do B" without providing any justification for it. Documents like this only contribute to holy wars between junior developers by creating an impression that they should blindly obey some random set rules rather than forming balanced opinions.

This document should rather be called:

"Some practices for JavaScript projects that work for devs at Hive but might not necessarily work for you."

> What in this document do you find useless or harming?

The use of TC39 proposals IN PRODUCTION. The authors of that document aren't serious.