Many software companies face challenges in their work with User eXperience (UX) and how to integrate UX practices into existing development processes. A better understanding of these challenges can help researchers and practitioners better address them. Existing research does not analyse UX challenges in relation to other software quality characteristics including usability. In this empirical study, we have interviewed 17 practitioners from eight software development companies. Their responses are coded and analysed with thematic analysis. We report 11 challenges that practitioners face in their work with UX. Some of these challenges partly overlap with those reported in existing literature about usability or software quality characteristics. In contrast to these overlaps, the participants of our study either view many of the challenges unique to UX, or more severe than for usability or other quality characteristics. Although at a superficial level challenges with UX and other quality characteristics overlap, we differentiate these challenges at a deeper level through two main aspects of UX: subjectivity and emergent nature. In particular, we identify at least five issues that are essential to the very nature of UX, and add at least seven extra difficulties to the work of practitioners. These difficulties can explain why practitioners perceive the challenges to be more severe than for other quality characteristics. Our findings can be useful for researchers in identifying industrially relevant research areas and for practitioners to learn from empirically investigated challenges and base their improvement efforts on such knowledge. Investigating the overlaps can help finding research areas not only for enhancing practice of UX but also software quality in general. It also makes it easier for practitioners to spot, better understand as well as find mitigation strategies for UX challenges.