Thank you Froggiz for this useful information on ncftp! Can't belive that vmware didn't add an integrated ftp client to their web interface, i mean cmon guys it's almost 2020!
I've tried using ncftpput to upload a vm files to a backup ftp but i kept getting a "miscellaneous error" while doing so.
Took me some time to find out that the transfer crashed when the upload speed was getting above 100 MB/s.
The problem was due to the default TCP/IP buffer size used by ncftpput that was not large enough for transfering at that speed, probably creating buffer overflows which generated the "miscellanous error".
Thanksfully ncftpput let you change the TCP/IP buffer size with the parameter -B as explained in ncftpput(1) manual page. After setting the TCP/IP buffer size to 999999 the error was gone.
Anyway this is a great way to save VMs on an esxi to a ftp server.