Since we're comparing the TOS Romulans and Klingons for traces of honor, let's look at their episodes.
The Klingons debut in "Errand of Mercy" by occupying a seemingly primitive planet and "executing" hundreds of Organians, all for Organia's strategic location against the Federation.
The Romulans debut in "Balance of Terror" by destroying several Earth outposts and killing dozens of Starfleet personnel, then attacking the Enterprise to test the Federation's military preparedness.
Both are evil aggressors. The Romulans have the one virtue of attacking only someone their own size.
The Klingons, though restricted by the Organian Peace Treaty (Star Trek's Mutual Assured Destruction), continue targeting less-advanced worlds by covert means that the TNG Klingons would call dishonorable. They foment a coup in "Friday's Child," poison grain meant for thousands of UFP colonists in "The Trouble With Tribbles," and they give the North Vietnamese superior firearms in "A Private Little War." Kang faces a little ball of hate instead of primitives in "Day of the Dove," but at least he knows when not to fight.
In later Romulan episodes, however, it's the Enterprise that violates the Neutral Zone, doing so in "The Enterprise Incident" to steal the cloaking device. The Romulans, though hostile, aren't even villains in these cases.
The Klingons were meant as the allegory for the Soviet Union, and so obviously it has more meaning if they change and make peace with the Federation. And Ron Moore or any writer can argue correctly that most societies will change over the course of a century, so the Klingons and Romulans can, too.
However, the portrayal of the 2 empires in TOS to me hinted more at eventual genuine understanding with the Romulans and a peace-through-comeuppance for the Klingons, like they got in James Blish's "Spock Must Die."
It's easy for me to imagine a Romulan version of Worf in TNG, exploring the martial side of Vulcanoids. He'd agree with the TOS Romulans that there's no honor in defeating the defenseless.
Instead of plotlines about the Klingon civil war, I suppose we'd have several episodes involving Worf with the Vulcan reunification movement, and episodes where the Klingons try to split the Federation/Star Empire alliance or the Romulans try to push the UFP into joining them in a war to destroy the Klingon Empire. And you still could have all 3 team up against the Dominion in Deep Space Nine.