This is why, like I said, I'd let the Romulans fly those ships but have them get costumed as Romulan ships.
That way, it becomes, "The Federation shared some design schematics for consoles and systems layouts", not "The Federation gave them ships."
But dstahl's answer thus far is that Romulans are basically now the equivalent of DS9 era Bajorans.
The Vor'cha was supposed to be a Klingon ship the Federation contributed design input, hence the grey hull.
I'd just prefer that Romulans flying Fed/Klingon ships get a designated Romulan 3d model to replace.the Fed or Klingon model, albeit with Fed/Klingon hull color options.
All Romulan Destroyers/Dreadnaughts would become Scimitars, which would mean rigging up some animations for the Scimitar. All cruisers would become D'Deridex, which would mean Cryptic would need to rig up a saucer sep. All Escorts would become Mogais, which would mean Cryptic would need to make the Mogai MVAM capable.
Again, the idea being that these are Romulan ships that fill the role of Fed or KDF ships, with Fed or KDF design input, rather than outright gifted ships.
But Federation ships are demonstratated to be WEAKER than Romulan ships
thats partially true, the d'deridex for instance was tougher than a galaxy, but it was a true warship that was 2 times larger, and even then, it was only a bit stronger, as for the scimitar, it was basically some kind of superprototype ridiculously heavly armed(and probably VERY expensive) when it first showed up on nemesis(romulans sure are fond of making superships), as for the valdore it was never showed to be stronger than the sovereign, it seemed to be equal to one at best.
the only true warships the feds have are the defiant and its variants(and the kumari variants if you wanna count it too).