The moment the /Z option showed up I figured there'd be an issue. I have to copy multi-TB files often using Robocopy. We originally used /Z because "Well, if it gets interrupted, I'd obviously want to restart from where I left off..." I accidentally left the switch off one day and noted it completed in less than half the normal time (of using the /Z flag).

In other words, yes, it may be restartable, but if it takes more than twice as long, just recopy the file(s) if needed.