If app. pool isn't stopped, and user (or come other agent) makes website filesystem changes after DB backup and initial files listing done by archiver utility, then 3 (well, at least) cases may happen:
1) If user adds new files in that time, archiver just not lists them, and DB backup (done before) just not contains references to them. In case of full restore needed, those newly added files will be lost - but it's OK, as some part of work will be lost anyway.
2) If user changes files content in that time, achiver will add changed files to backup, and DB already contains references to them (maybe with some desync about file size, etc.) So it also OK.
3) But if user deletes some files in that time, archiver already listed them (but they will not be added to filesystem backup, as there are no such files), but DB backup will still contain references to deleted files.
Point 3 is the main concern - how those references to non-existant files will be treated by DNN?