Pretty much all the C++ features I've used are good enough to write some toy code snippet, but it's hard to use them to good effect at scale without causing massive problems.

Even classes are an instance of this, they were to solve some perceived problems, but they created much bigger issues, such as readability issues and introducing many more compile time dependencies.

PIMPL wasn't even a C++ feature but an idiom pushed by some people. It is next to unusable because you have to duplicate the API and write all the call forwards.

One problem with std::unique_ptr for example is that there is no ergonomic way to use it to hide implementations. The reason is it relies on destructors and to use destructors the class definition needs to be visible.