It uses malloca, which uses global data without synchronization.  This can result in crashes or information leaks in multi-threaded programs:

  http://lists.gnu.org/archive/html/bug-gnulib/2018-01/msg00046.html

Furthermore, the following example program results in a valgrind warning:

#include <err.h>
#include <locale.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <uniconv.h>

int
main (void)
{
  if (setlocale (LC_ALL, "en_US.ISO-8859-1") == NULL)
    err (1, "setlocale");
  /* Try to provide better valgrind results.  */
  char *str = strdup ("non-idn.example");
  if (str == NULL)
    err (1, "strdup");
  unsigned char *result = u8_strconv_from_locale (str);
  if (result == NULL)
    errx (1, "u8_strconv_from_locale");
  printf ("[[%s]]\n", result);
  free (str);
  return 0;
}

==29579== Conditional jump or move depends on uninitialised value(s)
==29579==    at 0x4E4C61C: libunistring_freea (in /usr/lib64/libunistring.so.2.0.0)
==29579==    by 0x4E4EDA4: libunistring_mem_iconveha (in /usr/lib64/libunistring.so.2.0.0)
==29579==    by 0x4E55CFC: u8_conv_from_encoding (in /usr/lib64/libunistring.so.2.0.0)
==29579==    by 0x4E55FB7: u8_strconv_from_encoding (in /usr/lib64/libunistring.so.2.0.0)
==29579==    by 0x4006D9: main (unistring-uninit.c:17)
==29579==  Uninitialised value was created by a stack allocation
==29579==    at 0x4E4ED0C: libunistring_mem_iconveha (in /usr/lib64/libunistring.so.2.0.0)