IS NTFS-3G SAFE FOR MY IMAC?

I do not see any problem that prevents you from using it. NTFS-3G works properly in OS X, and you only have to worry about keeping it updated (if not, you may have problems with OS X upgrades because of its kernel extensions).

Another application you should consider is Paragon NTFS, although it is a paid application.

It is safe but IMHO not all that reliable. If you can I'd recommend a dedicated EHD for your Mac that is formatted in Mac OS Extended (Journaled).