yes, the pi is completely boring regarding the setup, all files simply sit on the vfat partition, OTOH i dont see a reason to update the dragonboard bootloader files (i doubt qualcomm has even changed them since the board was released).
if we just want to have a target for a complex update exercise the dragonboard is definitely the right device though.
regarding the changes ... the pi does all the HW initialization in the first stage bootloader blob and also ships additional features ...
is a very good example where power management features were changed ...
additionally the pi boot firmware also requires updating for major kernel version bumps and, given that the bootloader is actually the graphics driver, there can be driver related fixes too that you need to update for. specifically on the pi's the bootloader blob files are way more than a BIOS, while on other boards the BIOS comparison is actually true. in general, the update process here is simply "copy".
for UEFI we actually ship all bits and pieces from the fwupdate setup we also use on the normal ubuntu images, the process there should be identical and is AFAIK actually used on the dell gateways (which also use secureboot)