Since I have found little documentation on this issue, I wanted to
briefly summarize my results creating an UEFI-bootable Isohybrid-Image.
Just in case that perhaps they could be useful for someone.
Both packages work for me with the following differences:
*live-build package provided by Kali-Linux*
If you use "--bootloader syslinux" OR --bootloader is not set in
live-build configuration, both, BIOS and UEFI systems are booted with
Syslinux.
Advantage: That means more possible options and consistent
user-experience for BIOS/CSM and UEFI-Boot.
Disadvantage (for me): I use Isohybrid-Images, because ISO960 is
read-only and users can not accidentally delete files, when mounting the
removable device in another OS. To boot via UEFI, in addition to the
ISO 9660 file system there is a small - of course writable - FAT
partition needed. Different than if you use GRUB, using Syslinux it also
contains the /live folder with kernel and initrd, which therefore are no
longer read-only. This is a disadvantage for me, but for most cases, it
is probably irrelevant.
*live-build package provided in Debian Git*
If you use "--bootloader syslinux,grub-efi" OR --bootloader is not set
in live-build configuration, BIOS/CSM-Systems are booted with Syslinux
and UEFI systems are booted with GRUB. The latter works technically well
for me - even if it's not made for people with high aesthetic demands ;~}
Advantages and Disadvantages:What I wrote above the other way around.
Thanks to those who answered my questions.