It's honestly not really true on windows. Since for the longest time the msvc ABI was unstable people are used to interop through C APIs, or rebuilding everything. I personally build all my stuff with libc++ and it works fine.
If you also want to use libraries that are built against MS STL, then you can't really use libc++. Of course if you can build everything with libc++, that's fine. That's how Chromium is built, AFAIK.
That depends, on windows multiple standard libraries (even libc) can cohabit in the same process (but you have to be careful to, say, not free something in a different dll than the one which allocated it). If you are careful with that and aren't exchanging standard library types across library boundaries there won't be issued.
