  We argue that cubic order interactions between two scalar gravitons and one
tensor graviton are ubiquitous in models of dark energy where the strong
coupling scale is $\Lambda_3$. These interactions can potentially provide
efficient decay channels for gravitational waves. They can also lead to
gradient instabilities of the scalar perturbations in the presence of large
amplitude gravitational waves, e.g. those detected by LIGO/Virgo. In contrast
with models in scalar-tensor theories, there is an infinite number of higher
order interactions in generic $\Lambda_3$ models, which make it difficult to
predict the fate of these instabilities inferred from cubic order interactions.
