Which leaks a lot of implementation details through the public API, preventing changes without breaking existing clients. Which in practice means that you can't

Which leaks a lot of implementation details through the public API, preventing changes without breaking existing clients. Which in practice means that you can't change anything anymore once you have more than a handful of customers. At least if you don't have the market power of a facebook who can say "Adapt your client code or it'll stop working. We don't give a fuck." Another big issue is that it forces the client to understand a lot about how your application works. While an API can abstract that and conveniently offer various computed properties which output what the client needs.