Hi, on the question of UEFI versus BIOS, it's simply, they are newer 🙂 - For example, today, you would expect a modern motherboard for your home computer or Server for the DC to use UEFI, while you much older system from years ago may use BIOS. Same rules apply with these appliances.
In terms of the ISO, assuming you have the latest ISOMorphic, make sure you have the correct ISO for this appliance - you can find it on https://support.checkpoint.com/results/sk/sk181698 under downloads. Other "General" ISO's may not work. Also note, if used in Maestro, then there is a different ISO to use outside of Maestro. Regardless, you will see this in the downloads section description.
If still it wont boot in UEFI mode, then look for a different USB stick. I found that sometimes one that worked on an older appliance didn't work on a newer one (or vice versa). I ended up, I think with a few different SanDisk sticks for all the generations and things I needed to work with, and marked them up for their device "class". I'm travelling at the moment, so I cant check in the lab to give a specific model, sorry!
Tom