tldr

this works for me (without illegal byte sequence errors) regardless of file encoding.

cd /path/to/your/zip/files
mkdir combined
for archive in *.zip; do unar -f "$archive" -o combined; done

eli5

like others mentioned, the unzip on Mac is too old and maybe only handles utf8 well (thus incurring all the illegal byte sequence error). I tried upgrade upzip with brew install unzip and it didn't work for me.

remember to install unar if you don't have it already.

brew install unar