Auth is a big limitation. It's not a built-in component, they have an example [1] using the FastAPI layer for auth, but I haven't had the time to try implementing it. It's definitely not something you get out of the box with NiceGUI.
For scaling, I am viewing it mostly as an internal tool builder. I wouldn't recommend it for external applications. So as far as scaling an internal app I think it works fine. Their website [2] is built with NiceGUI, and it works fine, but you can feel the lag occasionally on some of their larger demo pages.
1. https://github.com/zauberzeug/nicegui/blob/main/examples/aut...