For retrieving files /folder from FTP via powerShell I wrote some functions, you can get even hidden stuff from FTP.
Example for getting all files which are not hidden in a specific folder:
Get-FtpChildItem -ftpFolderPath "ftp://myHost.com/root/leaf/" -userName "User" -password "pw" -hidden $false -File
Example for getting all folders(also hidden) in a specific folder:
Get-FtpChildItem -ftpFolderPath"ftp://myHost.com/root/leaf/" -userName "User" -password "pw" -Directory
You can just copy the functions from the following module without needing and 3rd library installing: https://github.com/AstralisSomnium/PowerShell-No-Library-Just-Functions/blob/master/FTPModule.ps1