I use SyncToy for a couple of years now to backup my data files. SyncToy "synchronizes" files so it is not a standard backup tool (i.e.: multiple versions of files, number of versions, automatic deletion of obsolete backups, ...). But for me, I only need an exact copy of my data files at two (or more) places, my computer and an external storage (external disk or lan directory on another computer in my house). SyncToy primary use was to backup (sync) photos, but there is no difference between photos and other files, these are files anyway.

When you update the same file at two places, I don't know what happens with SyncToy; I don't have the problem because I modify files at only one place.

«Are there any limits to file size, number of files within a folder, etc?» There is no limit, nor in file size, nor in the number of files within folders or sub-folders (I'ver never encounter none of these limits).

Hoping this will help...