The totally closed firmware and ROM on the m.2 nvme SSD is no more or less open than a modern wifi card with full open source Linux kernel driver support, which has a closed ROM running it.
Hard drives don't have that problem.
https://theintercept.com/2015/04/27/encrypting-laptop-like-m...
see the "attacks against disk encryption" section.
I think you’ll still have the problem with this thing being on a pci-e bus though.