There is a rational assumption built into some research projects that policy contexts are influenced by the quality of the evidence. This is, at best, only somewhat true some of the time. Through policy ethnographies, two education researchers working in the context of sustainability discuss their experiences with evidence-based policy. Central to both accounts is how critical messages about such issues as race, wellbeing and sustainability can become diluted and even lost. In the existing 'politics of unsustainability', and at a time of 'post-truth' politics, these accounts also show the limits of evidence-based policy. We argue that those working with 'the evidence' need to be open about how evidence-based approaches can end up supporting the 'status quo'. Moreover, while approaches such as knowledge mobilisation emphasise the relational qualities of policy contexts, and the importance of simple compelling narratives for decision-makers, they, like many other practices, do not sufficiently theorise the power structures surrounding knowledge and the policy context. In addition to the careful use of evidence, we argue that there needs to be greater emphasis on building healthy policy ecologies -- including far more emphasis on building critical and creative policy alternatives, especially in areas like sustainability and education.