I got a Vantec PCIe adapter, and installed a 250 GB Samsung 980 PRO (PCIe 4.0 NVMe M,2). It works well, after the driver loads, but I cannot boot from it. The problem is that it is mounted on a plug in card, that the BIOS doesn't know about, so it won't boot from it (because it doesn't know it is there). Air Boot also cannot "see" it, because BIOS doesn't know about it. Most (if not all) machines cannot boot from a plug in card, if the BIOS doesn't know about it. This is true for USB too.
If you have a machine, that knows about the adapter, it should boot from it.
Then again, it really doesn't make much sense to waste one of those things for boot code, unless you boot 14 times a day, or it is your only choice. I am still thinking about how to use it effectively. At least it is non volatile, so it remembers what was put on it, unlike what the RAMDISK does.