i have a viking machine which came with a good assortment of feet- some are plastic & some are metal- the plastic ones make it possible to see where you are going (can't see through the metal)
i would not however purchase feet that are not for my machine-no matter how cheap they are- i know some people do without incident- but i also some who have & done hundreds of dollars worth of damage to their machine- so i stick with feet that are for my machine- accept them in what ever material they are made from & pay what ever the cost is...just me...