Digital mental health interventions are routinely integrated into mental health services internationally, and can contribute to the reduction of the global mental health treatment gap that has been identified by the World Health Organisation. Research teams designing and delivering evaluations frequently invest substantial effort in deliberating on ethical and legal challenges around digital mental health interventions. In this article, we reflect on our own research experience of digital mental health intervention design and evaluation to identify eight of the most critical challenges that we or others have faced, and that have ethical or legal consequences. These are: 1) Harm caused by online recruitment work; 2) Monitoring of intervention safety; 3) Exclusion of specific demographic or clinical groups; 4) Inadequate robustness of effectiveness and cost-effectiveness findings; 5) Adequately conceptualising and supporting engagement and adherence; 6) Structural barriers to implementation; 7) Data protection and intellectual property; and 8) Regulatory ambiguity relating to digital mental health interventions that are medical devices. As we describe these challenges, we have highlighted serious consequences that can or have occurred, such as substantial delays to studies if regulations around Software as a Medical Device are not fully understood, or if regulations change substantially during the study lifecycle. Collectively, the challenges that we have identified highlight a substantial body of knowledge and expertise that is required, either within the team, or through access to external experts. Ensuring access to expertise requires careful planning, and adequate financial resources, for example to pay public contributors to engage debate on critical ethical issues, or to pay for legal opinions on regulatory issues. Access to such resources can be planned for on a per-study basis, and enabled through funding proposals. However, organisations regularly engaged in the development and evaluation of digital mental health interventions should consider creating or supporting structures such as advisory groups who can retain necessary competences, such as in medical devices regulation.