Two-dimensional (2D) electronic systems can exhibit dramatic drops in resistivity, xx, as the temperature T0, which has renewed debate as to whether 2D systems may have a metallic ground state. The metallic behaviour is only observed in low-disorder systems, where interactions between the carriers are strong. However, the strength of the metallic behaviour is not universal, with decreases in resistivity ranging from a few per cent to an order of magnitude as T0. Here, we compare the metallic behaviour in 2D systems dominated by long- and short-range disorder. We thereby highlight the pivotal role played by the type of disorder in determining the strength of metallic behaviour in these systems. In doing so, we resolve recent confusion in the literature that has arisen from the application of interaction-based theories of metallic behaviour to 2D systems whose disorder environments are inconsistent with the assumptions made by these theories.