If you want to have a USB stick with multiple bootable ISOs that can also hold any other files you wish, this is the way to go.
Firstly, once your USB drive is configured, it functions as any other USB drive for adding or removing files or folders.
Secondly, you can add or delete ISO files from any computer without having to run any program whatsoever.
UEFI and legacy boot compatible.
I have tried and tested this with all the most recent Ubuntu flavours/flavors including the GG Alphas.
Add any ISO by copy and paste from your file manager or delete one with your file manager.