Yes. In C++ in particular one of the most obvious ways to write unmaintainable C++ which such automation couldn't be expected to migrate is via abuse of the pr

Yes. In C++ in particular one of the most obvious ways to write unmaintainable C++ which such automation couldn't be expected to migrate is via abuse of the pre-processor. C++ retains the entire C pre-processor, which is a weird not-quite-language-agnostic text macro language on top of C. We can abuse this to write a different programming language, with different rules, and then at compile time the pre-processor transforms it into C++ which compiles. I'm confident a migration tool could not always usefully translate this, and I'd argue that's not a failing of the tool so much as an inevitable consequence of writing this unmaintainable code.