Ah, good ol’ perpetuation of bullshit.
Just like in Western medicine, sure, there exists a handful of quack Chiro’s that give the profession a bad name. And many who have not evolved with the research.
Modern Chiro’s are not just focused on spinal manipulation. They incorporate a diverse set of techniques, including a lot of PT rehabilitation overlap.
However… Some of us do require that temporary relief that only an adjustment can provide. I myself joint disorders that lead to an uncomfortable increase of pressure that needs release. I can pretty much ‘pop’ nearly every bone in my body. I hate it, but that’s how my body is. They are all different. Chiro’s have a place, maybe they don’t work for you, but they are Doctors, and for many of us, they provide relief that others cannot.
I use metal, but make sure it’s slotted otherwise you cake your dough up with flour/semolina/cornmeal. My personal go-to is semolina. You don’t need much and excess falls away more readily than flour.
Wood or metal, it’s more about technique at the end of the day.