Well, I have not done any research into safety, but I have three partitions:Oh, so you can partition it and format for the respective OS, and it works fine? I was under the impression it wasn't 'safe' to do so...
1.2 TB NTFS Partition for Windows System Images and File History backups
600 GB HFS+ Partition for Time Machine Backup
200 GB exFAT Partition for saving and sharing files between macOS and Windows
Both Windows 10 and macOS can read each others file system and the fact that Boot Camp NTFS partition can exist with HFS+ for years now; I don't see why it would be a problem on an external drive. It will be interesting to see how this is handled when Apple transitions macOS to its new Apple File System.