Streamlit Cloud is a free service intended for educational use and to provide an easy way to get started. There is a 1GB resource limit and apps will go to sleep if not visited regularly, requiring a prompt to reinitialize them. You are limited to having only one private app where you can control who accesses the published app. However, can have an unlimited number of public apps.
If you have an app that needs more memory and compute power, either because of handling large data objects or because of needing to support a large number of simultaneous users, you would need to look elsewhere. Additionally, there is no service level agreement (SLA) with Streamlit Cloud, so if you have something that is business critical, you would need to look elsewhere to make sure you had the appropriate guarantee on uptime.