Some of Rome undoubtedly is SFX, but I've just fast forwarded through an episode of I, Claudius to check: there's no scene that clearly wasn't shot in a studio. Even the exterior shots (market place, a garden) were done indoors, with the lighting turned up a bit. It's next on my 'to watch' list and I'd bet you never see the sky once in over ten hours.

In contrast, if the vast bulk of Rome exteriors aren't actually shot outside, I'd be amazed.

So this isn't about 'suspension of disbelief', this is like the difference before and after the invention of a movie camera you could move about.

Or the difference between a theatre play and a film. I, Claudius is recording of a magnificent play, Rome is a film.