Implementing new technology into a complex software developing environment comes with many challenges in terms of code, user interface design, and developer experience. It is well-established that the pressing demand for security and regulations within financial technology makes it even more essential to implement new technologies with care and minimised risks. This work aims to determine the effectiveness of such implementations. Specifically, it evaluates the effectiveness of implementing a UI library in a FinTech application to find the most suitable approach. In this context, a UI library defines as a robust set of user interface components available in one place and a FinTech application as an application in financial technology with complex developing infrastructure. In order to successfully evaluate the effectiveness of implementing a UI library in a FinTech application, a thorough literature survey was performed to identify decisive factors relating to code quality, user interface, and developer experience. In a case study in which a FinTech company served as an example, a solution consisting of their product and UI library was developed. The solution was tested by collecting data from code evaluation, questionnaires, and interviews. The result shows that the solution has higher code quality, fulfils the FinTech UI requirements, and perceived as an improvement of the development infrastructure.  On this basis, the utilised methodology and the recognised factors in this work should be taken into account to identify the most suitable approach when implementing a UI library in a FinTech application.