By definition, addition of the account of another to your credit report automatically results in any subsequent score no longer being representative of only your own personal credit history. That could become a possible problem.
If you apply for new credit, depending upon the type and amount, the creditor may or may not do a comprehensive manual review of your credit report as part of their approval process.
If the amount of requested credit is low, many creditors may rely only upon your three-digit score in their approval process,thus saving time and money. They thus will not be aware that the score they are using is not limited to an assessment of only the applicant's personal history.
AUs are thus often very useful for building or rebuilding.
Howver, as your score improves and you move up the credit ladder, there is usually increased liklihood that the creditor will do a manual review of your credit report as part of the approval process. Upon seeing presence of an AU account, they are aware that the score is not representative of only your own risk assessment. They may thus give your application more scrutiny. Some mortgage lendors have even been known to require removal of any AUs as part of their underwriting process in order to then obtain a "real" credit score based on assessment of only your own credit history.
