Generating a small (3x3) structure with natural floor and default settings worked fine, but I couldn't find a way for the Placer setting to generate a medium sized structure submerged in netherrack using move -x like in the overworld.
After reading the wiki and further testing it seems like the "match" setting gets ignored for larger structures?

Screenshots
I used insane spawn rates for the pictures but it looks the same when using the default frequent/common distributors, too.
I also tried to crank up the priority of the Match setting for natural floor and netherrack and it lead to no generation at all. Same goes for 1.0 match of netherrack and natural floor.

Not sure if this will solve your problem but it certainly generates a lot more reliably for me now. You'll have to test it in the next release.

It did solve the problem.