The language support might be nearly done but libc++ definitely does not have full C++20 support. For example, ranges, a HUGE feature in C++20, are disabled by default in libc++ (as of version 15, the latest stable release). As another example for a feature I want, std::source_location isn't implemented in libc++ even though it's existed in GCC/libstdc++ for many years now. In this example Clang has had the language support for implementing std::source_location for a while, but it hasn't been merged into libc++ for various reasons (last I checked there was a diff on phabricator that implemented it but there was some debate about the ABI that was being introduced, or something of that nature).

I don't think it's necessarily FUD, many C++ users don't understand the difference between core language features and standard library features. And even when you do understand the difference, with many features (e.g. std::source_location) you can't actually use the feature if libc++ doesn't have it, so the fact that clang itself supports the language feature is a moot point and irrelevant to end users.

I saw another comment explaining how to use libstdc++ with clang. Sure you can do this, but who actually wants to compile their code this way? Especially since it means you need to upgrade your compiler and standard library separately, on different release cadences. For most people this is way too much headache, they'd rather just not use the new features and wait until they're in libc++. You can also compile llvm with custom options to enable experimental features that aren't fully developed (like ranges), but who really wants to do this? Building a custom compiler/libc++ is a huge headache and the features are disabled by default for a reason anyway.

Just to be clear I love LLVM/Clang/libc++ and am huge advocates of them. But there's nothing wrong with pointing out flaws and where there are gaps compared to other implementations.