The flexible environment is optimal for applications with the following characteristics:
Source code that is written in a version of any of the supported programming languages:
Python, Java, Node.js, Go, Ruby, PHP, or .NET
Runs in a Docker container that includes a custom runtime or source code written in other programming languages.
Uses or depends on frameworks that include native code.
Accesses the resources or services of your Google Cloud Platform project that reside in the Compute Engine network.
You could also use both the environments to use it to your advantage.