The great sin of NEM is that there was so much at stake and the Federation-Romulan storyline held so much potential.
TNG had essentially been building up to a Rommie film since Season 1. The Rommies were the main power/political opposition in the broader TNG saga. It was a chance for TNG to go out with a bang.
The roots of the NEM fiasco actually went back to the end of First Contact
. After that film's success, TPTB should have sat down and mapped out a multi-film direction to wrap up TNG on the big screen in a big way.
Instead, we got the unmemorable INS and then tried to squeeze in a bunch of nonsense that made NEM such a incoherent and distasteful mess.