Dually registered advisors have different financial interests when acting as a broker because they are not required to follow a fiduciary standard. For example, brokers can be incentivized to promote certain products over others because they can receive a higher sales commission. Similarly, brokers can participate in a revenue sharing program.
Advisors registered with the SEC are held to a fiduciary standard (1), which means they are legally obligated to act solely in the best interest of their clients. Certified Financial Planners (CFPs) are also required to as a fiduciary.
Brokers registered with FINRA are only held to a “suitability” rule. They receive commissions based on the products they sell, not the ongoing advice they provide, and can recommend a transaction if there is a “reasonable basis to believe” it's suitable for the customer. (2)
Dually registered advisors act as both an advisor and a broker. Since they are registered with both the SEC and FINRA, they are more likely to have conflicts of interest because they can switch legal obligations at any time.