It all starts in Balance of Terror in TOS- the statements which sort of bind up everything that comes after in terms of the Romulans:
1. The notion that the Earth-Romulan war was fought with sublight ships and atomic weapons.
2. The notion that visual communications didn't exist at the time, so neither side ever saw what the other looked like.
3. The establishment of the Romulans as an ancient offshoot of the Vulcan culture.
4. The notion that 23rd Century Romulan ships are powered by 'simple impulse.'
Balance of Terror really was a bit of a mess to start with, as far as continuity was concerned. Many of the crucial/controversial points were put in the script to set up Kirk telling a crewman "We don't like racism in the future". The enemy had to look like a Vulcan, but it had to be a surprise to all the characters, so:
1) there were no visual comms in the war, 2) ships were destroyed to such an extreme that no-one even saw an enemy's dead body; 3) the Vulcans had no idea they were related to the Romulans, even though they look identical.
(Of course, TNG made the Romulans look quite different, but that issue seems to have been swept under the rug.)
There are also a couple of problems caused by use of terminology: "Their power is simple impulse" and "primitive atomic weapons".
The first one is obviously an error. We could handwave it by saying it wasn't refering to impulse engines, but to the actual power source of the ship, some sort of analog to a dilithium chamber.
"Atomic weapons" feels like a holdover from 50s sci-fi. The notion of ships firing nuclear weapons seems silly to us now. I think the best explanation is that they were using the word "atomic" to mean something different to what we expect. The actual word means "to do with atoms", so in theory it could apply to atomic disintegration, or a plasma cannon powered by an atomic reaction.
I think the way ENT approached the issue was really the only way it could be done. You can't have sublight ships dropping A-bombs on each other. The thing to stick with is the idea of Romulans being mysterious and tricky, and having a secret connection to Vulcan.
If ENT had stuck around long enough to go into the Romulan war, would they have had Federation people actually seeing Romulans? Probably yes. Which would of course been super-controversial.
But it could be explained as something kept secret in order to keep the Federation from falling apart, as one can imagine the Andorians would be even more distrustful of Vulcans if the truth was known.
I can imagine Shran being entrusted with the secret, and the terrible conflict it would cause in him.