The right technology is the technology that fits the job. Next.js can be a good choice for fast, responsive websites and web applications. Flask is a small Python framework that can provide an API when your product needs one.
Together, the two can keep responsibilities easy to understand: the web app handles the experience people use, and the API handles the data and business logic. That can be useful for a dashboard, a customer portal or an app with integrations.
This stack is not automatically the best option for every project. A simple business website may need far less. Start with the outcome, choose the simplest tools that can deliver it, and make sure the people who maintain the project can work comfortably with those tools.