I used powerISO to make my U3CUSTOM.ISO.
I didn't know the Universal_Customizer would make .ISO files.
lopacity, probably best not to hold shift if you are trying to have it enabled.
I don't know what affect it would have for certain (I haven't read the code),
but I do know holding shift is a feature for disabling some payloads.
Also, I would say it is safe to rely on safety.txt, but again, I don't know that for sure.