Macrium has suggested in the past that being able to backup to an FTP server is being considered.  I'd like to encourage it being moved up in the priority list.
In my case, my external NAS boxes are a pair of Synology DS212j's.  Like many low-end NAS devices, the SMB component that handles windows shares is not terribly efficient.  In my case, transfer rates with FTP are typically 4-5 times faster than with SMB.
At the moment I'm using ftpuse to map a drive letter to an ftp directory, but I'd really rather have Reflect handle it directly.
As a side benefit, if Reflect supported ftp directly, I could configure a NAS directory for my backups as accessible only via ftp, which would make it pretty thoroughly inaccessible to ransomware like cryptolocker.