Cal.com is awesome. We use it and frankly integrating just for site admins is just a drop-in. If, however, you think about building a module allowing booking pages and backend available to users, it would be a bit more complicated.

Cal.com is a Node/nextJS application. By default we don't ship Node with UNA. We do with NEO, however.

In generic integration the question would be what do people book... meetings with each other, events, classes? I can see how it can be integrated for a tailored implementation, but not clear how to do it in a generic way.