ISOmorphic works fine with USB 3 drives. It just writes to a raw block device. In fact, since it doesn't interact with the hardware directly, it doesn't have any way to tell if the drive is connected via USB 2, USB 3, FireWire, or SATA. It's even perfectly happy to write to an SD card in a PCIe adapter (no USB in the path).
In my experience, the only real limitation with ISOmorphic and USB 3 drives is related to the complexity of bootstrapping an x86/amd64 computer. Generally, a server which ships with a USB 3 controller on the motherboard also ships with a boot ROM which knows how to get a boot block from a device connected to a USB 3 controller. The boot block has the boot ROM get a kernel image from the device connected to the USB 3 controller using the boot ROM's drivers. Unfortunately, it's possible for the kernel image loaded in this way to lack drivers for the USB controller, so it fails to pull the rest of the data. This manifests as the installer being unable to find its own installation media.
Check Point has shipped extremely old kernels for a long time. Versions of Gaia up through R80.20 (released 2018-09-26) used kernel 2.6.18 (from 2006-09-20) which predates USB 3 (the standard was released 2008-11-17, with the first devices shipping over a year later), so it obviously doesn't have drivers to run USB 3 controllers. R80.30 (released 2019-05-07) started the transition to kernel 3.10 (from 2013-06-30), which finally added USB 3 drivers (most firewalls still used 2.6.18 until R80.40, in 2020-01-28). Now, R82 (released 2024-10-21) uses kernel 4.18 (from 2018-08-12), which has support for NVMe, among other things.