It varies. Literally everything from how modals work, notifications, etc. all had to be written. The things that I take for granted in my React app were fairly complex to figure out.

I know there are paradigms to handle that now, although the details get complex in practice. For example, the idea of a "portal" in react is actually really important for layering modals and popups properly, but it can be difficult in LiveView.

The biggest single challenge I remember hitting was dynamic recursive forms (for example a logic builder). It took a lot of effort to get right.