I do not agree. MaybeUninit<T> is without any doubt more valuable than the C FFI use
I can't even think of any prominent C FFI problems where I'd reach for the union's C representation. Too many languages can't handle that so it seems less useful at an FFI edge.
OS APIs for one, at least there are some Win32 calls that take unions if I remember correctly.
One of the reasons .NET had Managed C++, replaced by C++/CLI (nowadays C++20 compliant, minus modules), is exactly that P/Invoke (and RCW/CCW) cannot represent everything.
Which they don't want to expose on .NET type system directly.
I can't even think of any prominent C FFI problems where I'd reach for the union's C representation. Too many languages can't handle that so it seems less useful at an FFI edge.