Adaptive User Interfaces (AUIs) dynamically adjust to the user’s context, offering a promising path towards universal usability and enhanced User Experience (UX). While literature acknowledges challenges in designing AUIs, there is a lack of research identifying challenges from the perspective of designers with practical experience. Addressing this gap, this study investigates the AUI design difficulties by first systematising 17 challenges emerging in literature and categorising them across the phases of the Human-Centred Design (HCD) approach. Subsequently, interviews with designers with hands-on experience were conducted and analysed through a thematic analysis, revealing 21 challenges emerging in practice. Triangulating the findings from both sources revealed that six challenges identified in the literature were confirmed in interviews, while two were contradicted. In total, 32 design challenges were identified across all four stages of the HCD process. These findings provide a structured understanding of designers’ challenges when designing AUIs, laying the foundation for researchers and practitioners to develop strategies and methodologies addressing them.