Software startups operate in uncertain environments that require agile methodologies that support adaptability. Although Scrum and Kanban are widely adopted in software startups, their contributions to managing uncertainty and adaptability remain underexplored. This study investigates which aspects of Scrum and Kanban are the most effective in addressing uncertainty and volatility to improve adaptability in software startups. The goal is to identify key practices that improve agility and examine how startups tailor agile methodologies, particularly where they face limitations. To achieve this, a mixed methods approach was used, combining a literature review with a survey. The survey collected responses from 121 startup professionals to assess their experiences with Scrum and Kanban in handling uncertainty, workload management, workflow visualization, iteration planning, risk management, and testing. The results show that iterative development and a sustainable pace in it, prioritization, and visualization improve agility and productivity. Scrum supports structured iteration and events, while Kanban enhances workflow transparency and flow management. However, limitations in risk management, team structure, collaboration between business and technical people, and testing suggest agile frameworks require further adaptation in startups. In general, the study reinforces the need for context-specific adaptations of agile methods, as no single approach fully addresses the adaptability and uncertainty of startup environments. It highlights pivotal aspects of Scrum and Kanban to construct hybrid or customized approaches for complex, fast-paced software environments.