North Carolina is one of a handful of states that requires either the signature of a notary public or signatures by two witnesses in order to validate a mailed-in absentee ballot. It is among an even smaller number requiring the witnesses' personal addresses.
Information about the frequency with which these or other specific requirements lead to rejection isn't easily discoverable. One political scientist calculates that about 10% of ballots cast by mail in the 2016 general election were rejected. But absentees-by-mail amount to a small portion of voters, as many would-be absentees vote early in person. Overall, according to an MIT election database, rejected mail ballots amounted to less than 0.1% of 2016 ballots.
(Updated July 15, 2020: For the November election the state will require only one signature.)