I used a functional-reactive-language-that-compiles-to-javascript for a web app, in a project that lasted about 3 years. It solved callback hell, and solved some UI problems, but created some hard UI problems as well. I'm not sure how this would translate to a server, but some examples anyway.

It seemed impossible to completely escape imperative programming. Mouse click handlers for example were much more natural to write imperatively; changes made in the imperative code would propagate as signals in a reactive way.

Reasoning about what happened around the boundaries of imperative and reactive code was hard, especially as the application grew in complexity. If I have a UI element that depends on two other signals - think spreadsheet cell with a formula that depends on two other calculations - do I want to update it as soon as one signal changes? do I wait for both to change? Do I want different behaviors in different circumstances? It often led excessive layout changes as values passed through intermediate states, or code being executed multiple times unnecessarily.