But even fee-only advisors who forgo a commission and instead send clients to direct-sold plans may be leading them to invest dollars that might be better off in a tax-free Roth IRA. Some states, including California, New Jersey and North Carolina, don’t allow investors to deduct the money they contribute to a 529, negating one selling point of the plans. (There’s never a federal deduction.)
This Financial Planning.com article has a pretty good discussion of the pros and cons of buying 529 through a broker.