Difference between Update-CsUserdata and Import-CsUserdata
Update-CsUserdata does not overwrite existing data, it appends to it. Whereas, Import-CsUserdata overwrites the old data.
Update-CsUserdata is an on-the-fly operation and requires more intensive resources and thus, should be reserved for one off (non-bulk) user updates. Whereas, Import-CsUserdata is relatively faster and requires a RTCSRV.exe restart