Balance of Terror:
Constitutions-class vessel destroys the pride of the Romulan fleet in a running fight. Final tally: One Warbird gone, one fatality aboard
Enterprise.
The Deadly Years: An estimated ten Romulan warships don't manage to destroy the same
Constitution-class starship despite her being commanded by an officer highly inexperienced in space combat.
The Enterprise Incident: Three Romulan vessels of a design considered roughly equal to a
Constituion-class are outmanuevered by a single Federation ship which steals a cloaking device and gets away clean.
The Defector: Two Romulan
D'deridex-class vessels back down in the face of one
Galaxy-class and three unidentified Bird of Prey types (Probably
K'vort-class, much smaller than the
D'Deridex.
Klingon Civil War episodes (titled unremembered): Romulan task force refuses to fight Federation battle group when detected, despite the possibility of a Klingon Empire friendly to Romulan interests if the mission succeeds.
Unification Parts 1 & 2: Klingon vessel penetrates Romulan space as far as their capital without being detected. Romulans attempt quick take over of Vulcan, but destroy their own assault force when confronted with a single
Galaxy-class starship.
Deep Space Nine (episode unknown): Faced with the prospect of fighting the Federation fleet, Romulans remove weapons stockpile on Bajoran moon.
I can probably find more, but it makes little difference. Romulans have never been portrayed as an enemy as difficult to conquer as the Klingons, and judging from their reliance on skullduggery and ridiculous plots they're too weak to sustain a conflict with either the Federation or the Klingon Empire.
The Klingons, by contrast, did rather well against the Federation despite a few 'featured on Deep Space Nine' losses. Afterward, they still had plenty left to hold the line and were pivotal in several Alliance victories versus the Gamma Quadrant invaders. Also, in several 'alternate history' episodes we've seen, the Romulan Empire was subjugated by the Klingons....
In any case, it's kind of a silly thing to be arguing about. Go read some of my fanfic if you like Klingons and some of Jaeih's or Sethans if you like Rommies. Most of the people on that board are very good at capturing the flavor of their favorite Empires.