On the server you can run ASP.Net using Giraffe, which is a Functional Programming layer with comparable performance to C#.
On the front-end, you can write React in an true Functional Programming language.
And of course you can share F# code between frontend and backend.
This means that you can have a team of C# developers writing in a language they are familiar with, a team of node/TS developers writing React and a team of F# developers working on a pure functional core with all of the business logic. Write your validators in F# can you can share the same logic for a form in the UI and an API endpoint on the backend.
In my opinion having type inference, discriminated unions, computation expressions, et al., makes for a very concise and expressive way to model the core logic of an application.