I don't specifically remember a Romulan beating up Worf, but we have no reason to doubt that any given Romulan could, and as he's a Klingon that indicates the Romulans were stronger. Or at least more effective in the beating-people-up arts.
Worf actually defeats a burly Romulan in a hand-to-hand scuffle (after first disarming him with a surprise blow) at the end of "Redemption II". This after he has supposedly been weakened by torture.
We don't see Romulans fighting and defeating Worf or other Klingons. Typically, Romulans make sure they come in such overwhelming force that the enemy has no opportunity to fight back.
The differences between them are that the Romulans have changed from a pure stealth mode tactics like in the ENT and TOS days to having the largest ships in their fleet. The Romulan warbirds are huge and powerful.
We might also argue they are few in number, and used almost exclusively in the role described above: creating massive local superiority (despite overall inferiority) so that they very few fights the Romulans do pick will always end in their victory. That is, the lack of medium or small fighting vessels might indicate overall weakness.
In ST6, it seems Klingons and Feds stay out of each other's throats because Romulans might choose to ally with one or the other at an inopportune moment. But when Romulans enter into secret pacts, all-out war between the two superpowers suddenly becomes a possibility. (Perhaps the Romulans promised their allegiance to both