Afaik the only drawback is that this functionality is very new and still has some open issues (linked at the end of the post). As stated there are no dependencies for Zig and it is shipped in relatively small tarballs which can be downloaded from the Zig website: https://ziglang.org/download/

The limitation is maturity/stability I'd say. Zig is still pre-1.0

Speed? Using Zig should be faster than using Clang directly in many cases. You get the caching system, and I think you can do more complex builds without having to resort to multiple Clang commands from a makefile.

Not sure what you mean with external libraries.