In the context of the widespread integration of high-proportion renewable energy and power electronic devices, this paper analyzes the challenges faced by isolated synchronous power grids operating with high renewable penetration. First, the Irish, Texas, and Australian power grids are selected to review their system characteristics and renewable development paths, identifying frequency stability risks associated with high-penetration operation. Then, from five key perspectives: reduced system inertia, scarcity of frequency regulation resources, diversification of frequency regulation methods, increased risk of frequency oscillations, and the impact of distributed renewables on under-frequency load shedding, the common challenges faced by these grids are summarized. Finally, the systemic characteristics of frequency stability issues under high renewable penetration are identified, and future research directions are proposed, including critical inertia assessment, coordinated multi-source frequency regulation, oscillation suppression, and optimization of load shedding strategies. These insights aim to provide both a theoretical foundation and practical reference for building new power systems.