I've tried it a few months ago, and it works, depending on how you installed it
and i just tried using bunnycdn and it worked too
sample working config for bunnycdn:
1. in the domain and storage patch column you have to adjust according to the folder you created if you just go directly to the root ftp folder you don't need to add a sub directory to the url (example in my case, if you don't want to add a folder you can just fill in /gscnet ) adjust it to the ftp name you yourself, can be seen in the second picture.
uploading direct to External FTP/FTPS Server is working: