There are many instances where your research topic may not be best suited to performing a systematic review. You may not have the time, money, or team resources to perform one. Perhaps your question has not been well studied and you do not have a critical mass of studies to pool in a systematic review. There are other, just as valid, review methodologies you can write instead. Scoping and other types of advances evidence synthesis are used to analyze and inform research and research outputs.
An integrative review combines empirical and theoretical research to examine research on a given area. Includes non-experimental research, and can include case studies, observational studies, theories, guidelines, etc., and is generally used to inform healthcare policy and practice.