What if you forget to call “defer”? Then your code is incorrect. That’s kludgy in comparison to RAII where it’s impossible to forget to call a destructor.
RAII is the best for ensuring that things get cleaned up even if it does lead to more boilerplate to add the pattern to things. But even .NET's IDisposable feels better than defer. It's established practice to implement it if your class contains something that must be disposed. And it's easy for tools to check if you haven't disposed of something that implements IDisposable.
All that said, I find defer to be better than nothing. In C, the best I can do for cleanup is having all the cleanup functions called at the end with labels at the various points I need to jump to.
So it's immediately better compared to the current C situation.
As for compared to RAII? Well, thats one failure mode for defer (forgetting it), whereas there are dozens of ways to mess RAII...
In comparison to RAII, “defer” as a language feature is a kludge. Mainly because RAII completely solves the problem of correct resource cleanup for API users while “defer” does not.
So I don't see the comparison...
Destructors probably cannot be usefully imported into C while preserving the simplicity of the language: then you'll want at least unique_ptr to manage your memory with destructors, then probably shared_ptr, and some ownership semantics as you pass those around, et cetera. In that sense, defer strikes a balance between simplicity and usefulness. However, there is still a comparison.