Under the heading Pronouns: The Slightest Slip is Fatal, Jacques Barzun writes:
We now enter the dangerous wilderness of Pronouns. It is the duty of pronouns to be not wild but tamed, that is, tied down; yet their natural tendency is toward the jungle. At the same time, no decent prose is possible without the solid links afforded by pronouns with the right connections. […] Every pronoun necessarily has an antecedent. Which person or thing in the sentence that antecedent is must be immediately clear to the reader [p. 75, Simple & Direct: A Rhetoric for Writers, 1985, italics in the original].
Later, Barzun calls pronouns ‘the greatest difficulty in the writing of English prose,’ and that which is the most difficult pronoun to tame. I agree that pronouns are difficult, but in the end they are only a part of writing clearly and with style. In my opinion, clarity and style are the greatest difficulties in writing English prose (and perhaps in any language).
And while we’re talking of style, ‘it is’ is bad style according to Natalie Reid. The sequence ‘to be not wild but tamed’ also seems graceless. Better perhaps is ‘The duty of pronouns is to be tamed and not wild.’