Ok got it, i was missing some arguments.
For those in the same case, here are options needed :
See https://winscp.net/eng/docs/library_session_synchronizedirectories for details.
For those in the same case, here are options needed :
$session.SynchronizeDirectories([WinSCP.SynchronizationMode]::Remote, $Source, $Destination, $True, $False, [Winscp.SynchronizationCriteria]::Time, $transferOptions)
See https://winscp.net/eng/docs/library_session_synchronizedirectories for details.