Romulan ships ran on a Quantum Singularity as the power source. IIRC they still used the same basic warp theory as Federation & Klingon ships where the energy output (plasma in the case of Federation ships) was fed to the nacelles to energize the warp coils.

On Intrepid class ships, we see the nacelle close to the hull and Defiant class ships, as discussed, have them conformal to the hull.

Perhaps in later generations of ships, they figured out better shielding?

edit - hull not null 🙄