Sure man. I hear ya.
its 50/50 IMO.
and I am still a rookie here… not an end all be all preacher. 
Mild steel is much softer than 304 0r 321 stainless steel. Softer absorbs vibrations. Stress risers in poor welds can crack from vibrations.
Mild is much less brittle than stainless, not to say a 3/8 or 1/2" flange will crack on its own, but just stating that. The welds, bends, and everything else it stands to be true for also, so they could.
Mild is cheaper, as a material, and somewhat easier to machine… end result is a much cheaper flange.
Also keep in mind when you weld 304 stainless to mild, you should use 309 filler which has less chromium and can “over cook” easier when welding and loose the “stainless” ability and rust. Furthermore since 304/321 mild steel expand and contract differently. Again, which could lead to cracking. Think fiberglass body kits molded on steel body panels, they crack in the sun if not done right.
thats my 24.2 peso’s