And I disagree Too Much Fun that the Romulans were squandered as antagonists when they were directly behind the evil schemes in "Redemption" and ["Unification"], they also seemed to be pulling the strings in The Undiscovered Country. "The Neutral Zone" was a boring and patronizing episode that only introduced the Romulans in the last five fucking minutes, most other Romulan stories after that were a big improvement.
See, you mentioned
"Unification" as a counterpoint to my argument that the Romulans were squandered in TNG, but I see it as proving my point. I understand where you're coming from about
"The Neutral Zone". I don't deny that it's not very impressive as a major Romulan episode, it's really more about that silly time travelers plot...my point is that those last
"five fucking minutes" 
did a great job of setting them up as a potential major threat in future episodes/seasons, and I don't believe the show ever delivered on that promise.
There were a scant few episodes in which they did make excellent villains, but I think in general they were shunted to the side as the Borg became the more prominent recurring foe, and the Enterprise was more frequently threatened in more significant fashion by one-shot antagonists. I personally think
"Unification" did a bit of a disservice to Romulans as a respectable adversary. Sela was made to look incompetant and easily outsmarted, which in turn reflected badly on those Romulans under her command.
I see it less as an episode that re-establishes the Romulans as a powerful enemy and more as one that weakens them by being an episode that exists less to flesh them out than to squeeze guest spots by Leonard Nimoy and Denise Crosby into the show. I also think the most dramatic scenes and the most effective plotline in the whole two-parter didn't involve Romulans at all, but rather Data and the 'android prejudice' he faced in his first ship-running assignment.