There is ongoing discussion worldwide on the production of green hydrogen at scale within hydrogen hubs which integrate renewable energy resources (RES) and large-scale hydrogen electrolysis technologies within microgrids, co-located with hydrogen storage, distribution, end-use processes. One of the key challenges in development of hydrogen hubs relates to maintaining their grid stability and balancing with variable RES. To reduce capital costs associated with additional grid enhancing technologies such as batteries and synchronous condensers, there is a discussion on leveraging electrolyzer flexibility to enhance microgrid stability and minimize renewable energy curtailment. In the context of this work, flexible operation of electrolyzer refers to adjustment of its active and reactive power to contribute to microgrid voltage/frequency stability, also managing renewable energy output. This work aims to highlight the potential electrochemical and operational challenges that may limit the flexible operation of electrolysis plants and their participation in system support services, such as stack degradation, reduction in hydrogen production efficiency, stack ramp rate limit, stack partial loading constraint, and the challenges in no-load operation of electrolysis stack. Additionally, we clarify various operational constraints imposed by the downstream hydrogen storage/process that may limit frequency control from electrolyzers. Our comprehensive analysis seeks to provide practical recommendations on electrolyzer utilization for system support services, ultimately adding certainty to design and development of hydrogen projects.