Well, not exactly. More like if you WANT a race that is supposed to be "the most common" that isn't humans, dragonborn are probably your best bet, because they actually have some measurable advantages against us.
I mentioned nutrition above. Nutrition was the other incredibly serious issue for most humans across most of our history, and those issues are going to be much bigger for dragonborn, because they need more protein than we do. Protein is, unfortunately, harder to get than carbs or fat, and meat is by far the best source of protein in a pre-industrial society. Ironically, having a very high-protein diet could actually protect them from many of the problems that were induced by humans transitioning to settled existence (namely, dental issues) but likely would introduce other issues unless, as stated, the fact that they're physiologically different from mammals would protect them from animal-derived pathogens.
Setting up new food sources, particularly if they're travelling a long distance from somewhere else, will always be a tricky thing, depending on exactly how high-protein their diet needs to be. E.g. humans generally shouldn't eat more than about 35% of their daily calories as protein; the implication from the Ecology of the Dragonborn Dragon Mag article is that their diet would be closer to a minimum of 25% protein and a maximum of, say, 50%. If that's the case, their expansion will be limited either to places that already suit whatever agricultural methods they use (e.g. herding for inland populations, fishing and other aquaculture for people living near water), which is less a "secret weakness" and more an obvious logistics issue. Likewise, they'd need to develop preservation techniques for keeping enough meat on hand to feat their militaries, so while they have the manpower to wage war, they may struggle with maintaining logistics and supplies for war.
Locations with natural caves and thriving fish/shellfish sources would be prime targets for settlement, as caves would form naturally-defensible nesting/incubation sites, and fish provide a ready source of high-protein food. Mountains and deserts would present major difficulties for expansion, so human groups wanting to prevent dragonborn from spreading in and (potentially) taking over would want to button up any valleys or passes SUPER hard, and might even want to thin the numbers of any livestock-adjacent animals in their territory to deny possible food sources. More or less, prevention would be the watchword, rather than trying to take them on directly once they've gotten a solid foothold.
TL;DR: Because raising (or fishing up) lots of meat is hard, dragonborn will be more vulnerable than humans right at the initial founding of a settlement, when their logistics aren't well-established and they're the most at risk of Stuff Going Wrong. Kind of "in for a penny, in for a pound." Once built up, they'll be a nightmare to remove; likewise, if they're not trying to build up, their individual strengths give them an edge. But when they're invested but haven't yet gotten the infrastructure in place, they're extra vulnerable: no incubation sites, livestock not yet well-protected, poor road networks for receiving support from the homeland, etc. Terrain features would be better defenses than fortifications or armies.
