This advice came from a panel with Kristan Higgins (although I heard the same thing repeated in two other sessions).
There’s a fine line between a character who’s strong and a character who’s nasty. Readers are much less forgiving of the latter when the character is female.
I don’t like that this is true, but I totally agree that it is, both in books and in life. We want to write strong, female characters, and yet if we make them too strong, they become unsympathetic.