You might not know exactly what you're building. Maybe you vaguely know the fundamental idea. Maybe you know the problem to solve, but never formalised the actual business process. The fact that you're quickly up and running and able to experiment allows you to easily prototype, discover and crystallize ideas. It's not obvious at this point, but in the end you will NOT have to throw the prototype away and start over.

There IS a bit of a learning curve for any new technology. While Metapsi is no different, there's just a small set of unique concepts it introduces. Otherwise, the framework is focused on exposing the WEB standards: you work with HTTP, HTML, JavaScript and CSS, the only difference being the application is only driven from C#, not from each of these separately. This makes existing knowledge compatible with the technology stack, so every team member will find the right place when onboarding the project.

Since everything is C# you don't lose the familiar tools like Visual Studio / VS Code. The .NET solution will maybe be split in projects. If your team gets really large, you might even distribute pieces of the application as Nuget packages. In the end it's just C# that outputs HTML, JavaScript and CSS.

While Metapsi is flexible and follows web standards, it also has an opinion about one thing: The implicit architecture is model-based. Not only this ties server and client together, it also allows you to retune to performance by simplifying distribution of workloads and caching.