It lacks type safety because it's not possible to distinguish between an sc_array and a pointer so there's no way of detecting that someone passed a char * that nobody had ever called sc_array_create on to sc_array_add for example.
The alignment is broken because nothing in the C standard guarantees that the elems member of sc_array will be aligned correctly for any possible element type.
I also spotted another problem, in sc_array_init the code `void *p = a` is also not guaranteed to work. In an example snippet such as `int iv; sc_array_create(iv, 0);` expands to `sc_array_init(&iv, sizeof iv, 0)` so the type of the expression `&iv` is `int *` which is then being converted to `void *` in the function which is actually not allowed by the standard. This is also the reason why if you were writing a wrapper around realloc which exited if the allocation failed you would still have to pass in the current pointer with void * and return the new pointer with void *. This could be applied here actually as an easy fix but it indicates even further to me that the author of the library is taking a very leisurely approach to writing conforming C. This pattern also appears in the other two functions though and I'm not sure if in those cases it's something which can be easily fixed.