Mergers of compact objects, such as black holes and neutron stars, have been nicknamed standard sirens, by analogy with electromagnetic standard candles, because their waveform directly gives access to their distance. When an electromagnetic counterpart is observed, such sources thus allow us to construct a Hubble diagram, just as supernovae. Recently, the gravitational-wave Hubble diagram has been argued to be a key probe of alternative theories of gravity, such as Horndeski models. In this talk, I will discuss the foundations of this idea, and its limitations when the inhomogeneities of our Universe are taken into account.