Indeed! Which is why the only portable solution is to not do this.
For stuff that needs to be bootable by both BIOS and UEFI the only portable solution is to use MBR, not GPT. That means all legacy BIOS systems will boot it, and so will all UEFI systems since UEFI must support MBR.
For ISOs that need to additionally be booted off of optical media (aka ISOHYBRIDs) the story gets more complicated, but ultimately what you need to take away from that is the same: avoid GPT at all cost.
For stuff that needs to be bootable by both BIOS and UEFI the only portable solution is to use MBR, not GPT. That means all legacy BIOS systems will boot it, and so will all UEFI systems since UEFI must support MBR.
For ISOs that need to additionally be booted off of optical media (aka ISOHYBRIDs) the story gets more complicated, but ultimately what you need to take away from that is the same: avoid GPT at all cost.