The Business-to-Business (B2B) digital platform ecosystem developed in the Advanced Multimodal Marketplace for Low-Emission and Energy Transportation (ADMIRAL) project has significant potential to optimize multimodal logistics operations, drive digital transformation across supply chains, and promote emissions reduction. However, the effective implementation of these ecosystems often depends on overcoming challenges related to collaboration among stakeholders. Research on the potential challenges they face remains relatively limited. This paper aims to investigating the barriers and enablers of horizontal and vertical collaboration and assessing their importance to three groups of the ADMIRAL platform ecosystem stakeholders (sellers, buyers, complementors). We employed a three-step approach: (1) conducting a focused literature review to identify barriers and enablers of both vertical and horizontal collaboration, (2) surveying representatives from three stakeholder groups within the ADMIRAL platform ecosystem to evaluate the significance of those factors, and (3) utilizing the Distance-based Analytic Hierarchy Process (DAHP) to prioritize them. The most significant barriers to both types of collaboration are issues related to information-sharing resistance and cost appraisal. In contrast, cultural and language differences are seen as the least important obstacles. On the other hand, the top enablers of collaboration include regulatory and legislative compliance, as well as improved operational efficiency and productivity. Better global market positioning is considered a lower priority in both forms of collaboration.