(Familiarity with React and Typescript is recommended)
The biggest benefit of OneJS is the ability to design your UI with the React (JSX) workflow. We achieve this by providing a thin layer of "dom" for Preact. Under the hood, it's just pure UIElements, so things will be very performant. UI Toolkit's controls, events, vector API, and styles all just work out of the box.
With OneJS, there's no more C# scripts to compile everytime you change some UI code. You will have super fast iteration time, especially with the built-in Live Reload.
Besides UI, you can also easily use OneJS to build Addon systems for your players, so they can also have the power of Typescript + JSX + CSS to create addons/mods/contents for your game.