My biggest difficulty with fusion as a power source for warp is that less than 1% of a fusion fuel's mass is turned into energy when burned. And in the case of single, uncatylized deuteium fusion, it's less than 0.1%. That translates into 100 - 1000 times the mass in fuel for the same range at any given warp factor compared to M/AM. And I'm not sure there's room for that much tankage even if we assume the nacelles of the BOP are the same size as those of Enterprise.
Perhaps with our limited 21st Century understanding of the physics involved. The Romulans may have had thousands of years to get it to work well enough to be a good choice for their needs. No doubt, their fusion reactors have the same historical roots as the Vulcans which no doubt are at least conceptually represented in the fusion reactors in modern Federation impulse drives.
For my money, "Real Science" has only a back-seat informative role when massaging the numbers for the magic we see in shows like Star Trek