I haven't researched this in much detail, but I believe mobile deposits are treated as substitute checks under the Check Clearing for the 21st Century Act. This means they are subject to the same requirements as checks under Regulation CC. You might also want to think about the potential UDAAP implications of placing holds for using non-branch deposits encouraged by the bank, assuming your bank encourages mobile deposits.
One way to manage risk is to adopt limits on the dollar amount that can be deposited via mobile deposit or to limit mobile deposits for new accounts. The other is to review the deposit risk factors in a fashion similar to what you use for other check deposits and determine whether to place a hold.
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Opinions are strictly my own, and have nothing to do with my employer.