You hit on a great point. The possibility to be coerced is pretty much the single biggest part of the entire security investigation. It includes being coerced because:
-Financial debt, so all of your finances are inspected
-Family connections, so all of your family is investigated
-Marital affairs, so you're asked about it during the poly
-Pirating software, again asked about it on the poly
...and all sorts of other things. The big thing you hit on is that this isn't very painful unless you try to hide it. If you try to hide something and it comes out in the investigation, you'll almost certainly be disqualified.
One thing I learned in the briefings I saw after getting my clearance was that the single biggest motivator for betrayal was a thrill-seeking narcissistic personality, followed closely by political agendas. Financial (including bribery/blackmail) and romantic blackmail concerns were so tenuously correlated as to be laughable to suggest they are meaningful as a potential exploitation.
-Financial debt, so all of your finances are inspected
-Family connections, so all of your family is investigated
-Marital affairs, so you're asked about it during the poly
-Pirating software, again asked about it on the poly
...and all sorts of other things. The big thing you hit on is that this isn't very painful unless you try to hide it. If you try to hide something and it comes out in the investigation, you'll almost certainly be disqualified.