  Gravitational wave observations of compact binaries allow us to test general
relativity (and modifications thereof) in the strong and highly-dynamical field
regime of gravity. Here we confront two extensions to general relativity,
dynamical Chern-Simons and Einstein-dilaton-Gauss-Bonnet theories, against the
gravitational wave sources from the GWTC-1 and GWTC-2 catalogs by the
LIGO-Virgo Collaboration. By stacking the posterior of individual events, we
strengthen the constraint on the square root of the coupling parameter in
Einstein-dilaton-Gauss-Bonnet gravity to $\sqrt{\alpha_{\rm \tiny EdGB}} < 1.7$
km, but we are unable to place meaningful constraints on dynamical Chern-Simons
gravity. Importantly, we also show that our bounds are robust to (i) the choice
of general-relativity base waveform model, upon which we add modifications,
(ii) unknown higher post-Newtonian order terms in the modifications to general
relativity, (iii) the small-coupling approximation, and (iv) uncertainties on
the nature of the constituent compact objects.
