You have tried both sending a functional drive and an image previously? Or have you only sent an image?
I cannot really comment on this as I have never had any computer fail to boot from a DFSEE cloned drive that is within the boot specifications of the target computer in regards to disk size or geometry.
What occasionally happens is that if I send a partition image to a third party they will incorrectly copy the partition onto a drive with incorrect partition settings, the usual partition not marked active or bootable etc, incorrect alignment on an SSD etc.
I have solved this either by sending them a compressed disk image for a drive slightly smaller than the drive that is going to be used on the other end, rather than a partition image, this ensures that the partitions are written to the drive as found. The other option is to make them install Air-Boot as it can force boot the image regardless of it is partition settings.
The only other problems I have ever encountered are embedded BIOS's from a company called Insyde, used in Axiom industrial computers and 10Zig (Clientron) Thin Clients for instance. They both have incompatibilities with standard BIOSes when it comes to booting especially if they are used in conjunction with eMMC and also weird BIOS boot lockdowns intended to ensure you only use HDD's and software from the original supplier.
But as I said, I regularly clone OS/2 drives and partitions here for testing back and forth on hardware ranging from Cyrix based thin clients from the late 90's with tiny featureless embedded BIOSEs , to modern Xeon hardware with UEFI and never seem to have any problems going backwards and forwards regardless of hardware, with the obvious caveats about configuration setting.
There may be other options available, send them a VBox image converted to raw, transfer that to hd using rufus? YMMV but works for me with some caveats ...