I've heard e-fuses in general are vulnerable to optical inspection under polarized light after deliding a part. So if someone capable really wanted to clone a device, it's very possible they already were able to get the e-fuse key values.

I once used the e-fuse feature of another part for bootloader integrity. I wasn't worried about encryption, but the part would validate the bootloader integrity when encrypted. If integrity failed, the part would keep searching for a valid image. It was an easy way for some protection against flash corruption.