Stakeholder engagement is increasingly required in environmental research and governance, yet significant challenges remain in translating participatory ideals into research projects practice. This paper examines stakeholder engagement across 15 European case sites that used Multi-Actor Reference Groups and participatory modelling to support the assessment of Natural Small Water Retention Measures. Drawing on interviews with local research teams, observations, and monitoring of stakeholder interactions, stakeholder continuity and the added value of stakeholder engagement were explored and the extent to which the engagement processes were fair and effective was investigated. The results show that stakeholder engagement was shaped by a series of trade-offs that limited the simultaneous achievement of multiple engagement objectives. Broad and representative participation increased the diversity of perspectives but often reduced opportunities for building strong relationships, trust, and sustained involvement. Continuous high levels of engagement for all stakeholders during the project lifecycle, referred to as important for enabling active engagement, proved difficult to secure in most cases. Inclusive engagement implied involvement of a relatively high number of stakeholders, but this resulted in fewer opportunities for learning across actors. The findings demonstrate that meaningful stakeholder engagement in environmental research projects implies navigating competing demands related to continuity, inclusiveness, influence, scientific rigour, and mutual learning. Different strategies to overcome constraints for meaningful engagement were undertaken adapting to local contexts. The findings highlight the need for flexible engagement schemes that responded to stakeholders’ availability, and systematic evaluation of engagement processes to strengthen learning, legitimacy, and project outcomes.