With NEM, the broad consensus seems to be a total re-write would have been necessary to save it.
The entire plot and underlying thematic focus on the nature/nurture debate were completely out of place for TNG's last adventure.
-Shinzon as a Romulan Julius Caesar, indifferent to a small fish like Picard.
-Riker and Troi already married, already in command of the Titan (off-screen character growth and narrative development worked just fine in the TOS films).
-No Earth destroying weapon
-No Sela (!)
-Allow Worf to be separated from the main plot and then worked into the main events in a believable way rather than just lazily throwing him back on the Ent bridge.
-Add Spock and make it a conclusion to the Unification storyline. Give it some scope.
NEM should have been TNG's capstone epic story. All the elements were there. In retrospect, the seeds of NEM's demise were probably planted when they greenlit INS. That meandering film added precious little to the TNG story and forced the final film to cover more ground than should have been expected. And then it went downhill from there.