No, not really. It works quite well for large projects too. Just look at any open source that is C-based.

The only mechanism for safe-guarding access to private parts is using privately defined structs that are passed around in a form of opaque pointers. Some project use it (PGPphone did, the original SSH did, etc.), some don't, e.g. BSD and Linux kernels.

It's really more of a matter of developers exercising some basic thought when using other people's code. And it just happens so that there's more of them outside of the C++ group than in it.