Colleges using CSS Profile and the College Board NPC could make the NPC quite accurate.
However, for divorced parent situations at colleges that require both parents’ finances, whoever uses the NPC needs to have correct and accurate information for both parents’ finances to put into the NPC. Unless they trust each other fully with their finances (unlikely), or they can have a trusted-by-both neutral third party do the NPC, the NPC runs with accurate information for one parent and incomplete or guessed information for the other parent may not be accurate.
While the CSS Profile itself allows each parent to enter information without the other parent seeing it, confidentiality cannot be assumed, because someone who knows one parent’s finances and the eventual actual financial aid offer can reverse-engineer a range of values for the other parent’s finances (using the NPC).