I believe you put the word NBC in the mouth of whoever gave you the feedback from your congress person's office, that's why I wanted to know precisely what you told them. I know for a fact USCIS typically wouldn't respond to a congressional inquiry by telling them a case is pending because the NBC declined an expedite request. What they typically say is the pending background check is responsible for the delay a petitioner is experiencing.
Okay, first of all you need to understand the NBC process is part of the overall USCIS process. Your FO CANNOT and WILL NOT approve your petition if your background check is pending. The FBI background check is all part of the USCIS process. So there's no such thing as if the USCIS is holding your case, you think you should go through, since you've been through several screening you previously went through. And I'm sure the FBI was also involved at one point or the other in the several screening you previously went through.
Be prepared in the sense that if you've been through AP with your NIV, your IV petition will also be subjected to a prolonged AP. Same thing for those who experienced prolonged AP for during the IV petition process, they will equally be subjected to a prolonged AP when they apply for citizenship. Clearing a prolonged AP process does not automatically mean that is the end of the story, it's bound to happen over and over every time to apply for some sort of immigration benefit.