Flexible Triboelectric Nanogenerators (F-TENGs) are rapidly emerging as highly promising devices for energy harvesting and wireless sensor applications, offering self-powering capabilities and advanced smart sensing functionalities. However, their performance is contingent upon various influential factors that necessitate diligent attention and resolution. This review presents a comprehensive analysis of the factors influencing the performance of F-TENGs and highlights the recent advancements in self-powered sensing technology utilizing them. The initial provides a concise overview of the fundamental concepts and working mechanisms of F-TENGs, critically assessing the impact of various factors such as materials, surface morphology, environmental conditions, and power management units on their performance. These advancements have the potential to significantly broaden the applications of F-TENGs across diverse smart sensing domains, contributing to sustainable development and wider end-use. Moreover, the review underscores their crucial role in self-powered energy devices and their potential as integral sensing elements for Internet of Things (IoT) platforms. It also explores the challenges and future directions of F-TENGs, providing a roadmap for their development, scalability, and commercialization within modern IoT and AI frameworks. Ultimately, this review offers an in-depth understanding of the factors influencing the output performance of F-TENGs and proposes strategies for future research to unlock their full potential, enabling stable and autonomous smart sensing solutions.