Pull resistance - when I learned about the butterfly knot, I also learned the term 'spilling', because the butterfly doesn't spill in any direction (which I used to great effect on a long lead for a very energetic puppy). Wikipedia says it's also called 'capsizing'. Interesting that the author discovered the butterfly knot had this quality without learning either term.

Seems to me that another quality would also matter: length. A butterfly knot takes a good bit of length out of the rope, which shortens your longest sentence.

He did encounter another quality I always found quite frustrating: knots that can't be tied in the bight. Usually when I'm doing knots I need at least two. If you tie the wrong one first, you start fishing the standing end through in order to tie the second, before you realize that it would be faster to start over.