Let us assume that the reasons for additional review are due to red flags in your application. For instance, you know that while you do meet all the requirements and have submitted a complete and accurate application, you do have problem areas like criminal history and/or have submitted affidavits instead of documents asked for and/or you have questionable details. In that case, until the USCIS has determined without a reasonable doubt that you are eligible, you will need to wait.
Needless to say that if you have major issues with your application, and are not eligible, or filed the application with false information, your application will be examined more thoroughly. And finally, if you are not eligible for approval and are actually a candidate who has been flagged for deportation, the delay in a decision is a sign that the agencies are simply completing the steps needed for removal proceedings.