Wineries in France and elsewhere have used concrete tanks for at least a century, drawing the inspiration for such vessels from antiquity, when clay or ceramic containers were used for wine Over the past half century, stainless steel tanks have pushed out both concrete and wood fermenters (although wood is making a comeback for premium wines).
FYI, stainless rusts eventually as well.
I wonder if they have rebar in them? If they do, it will rust, then leach back into the liquid. Or they could epoxy coat, use fiber reinforcing, etc. I'll stick with glass.