I think you can build pretty complex applications, especially suited for the enterprise. However, don’t expect to build apps with rich user experiences (hence why I say they’re more suited to the enterprise). “Rich user experiences” is a very subjective comment. Streamlit has great charting components, for example, but no native cross-widget communications, and limited layout responsiveness, so your milage may vary (folks in this forum have built some very interesting Streamlit components/frameworks that are changing this situation).
As apps get larger they will benefit from partitioning and modularization, such as: multiple pages, database offload, separating front end vs back end concerns, and perhaps even wrapping back end code with APIs (especially if you’re integrating with other systems or running async workloads). In most cases big apps will need to be deployed in other public clouds, not the free Streamlit Cloud, which has limited resources.