Re: N minutes later, your files are also available on the remote Synology.
Where N can be a rather large number if there's a WAN in between the two*. ...and this is it, a good product should tell you that though you have commanded it to transmit 1Tb of files across a WAN, these are the ones that have made it across the link so far. This is not a criticism of the hardware, but a fact of life concerning the speed of light through a typical broadband connection.
*Which could be a life-saver if you've just been hit with Ransomware, but it is prudent to ensure versioning is available on your chosen product, and that the versions are easily accessible. All very well having the pre-ransomware version of the 1000 files so far encrypted in there, but not if you have to restore them individually.