IMHO the biggest advantage that Debian/kFreeBSD would have had would be first-class ZFS support. You can use ZFS with Debian today, but the license problem means it only gets supported through DKMS, which is a pain; a FreeBSD-based Debian could ship binary packages for ZFS that just worked out of the box.
Considering the amount of Linux distros based on Debian, it is truly a shame that Debian/kFreeBSD stagnated. We could have had Proxmox/kFreeBSD with support for ZFS Boot environments! The evolution of FreeNAS/TrueNAS demonstrates things have been going in the opposite direction.