> Zero terminated strings are only used in C so every other language needs to do a copy to pass a string to C, even C++ (for string_view at least).
This is one of the most frequent complaints and I find it ridiculous. C has string literals that encode zero-terminated strings, but you don't have to rely on these zero terminators. There are a few awquard "string functions" in libc, most of which you should just ignore. Besides maybe the printf() family (which you probably wouldn't want to use for interoperability there isn't anything that requires zero-terminated strings.
This language gives you memory to work with, and it's up to you how you want to achieve your goal.
> A lack of fat pointers means that buffers need to pass their pointer and length separately, which is awkward, or use a custom struct which other languages will know nothing about.
It could be considered a security problem that sizes have to be passed separately, but what I consider awkward is actually the slices approach: In general the count field in a "slice" is redundant data - think about parallel arrays, where with slices you need to synchronize the count information across all slices. Passing separate length is the correct approach from a normalization standpoint.
> It has no standardized error handling mechanism to help with writing automatic wrappers on the other language endpoints.
Similar to the point about slices, not including any notion of "errors" is the correct way of doing it. In general you don't want errors in a system, only data that you handle in an appropriate way. Also, returning what people usually consider errors is often a bad idea, as such error information should better be stored in a persistent handle or context struct.
> It has no modules or namespacing.
Not a problem, since there are names that you can already use for disambiguation. Namespaces only add more ways to identify the same object, which leads to ambiguity and/or hard to read and hard to grep code. (Namespaces encourage the use of not-fully-qualified identifiers - actually this is the only thing they really add, and apart from some metaprogramming tricks you can play I don't think it's a great addition).