from my experience, even with awesome welding and materials, resonance/vibration can cause failure on thick or thin wall manifolds/headers in a short amount of time...been there done that and it sucks. however, if you design the system to perform outside of these modes, it will last a lot longer. if in any doubt, stock OEM cast manifolds are beastly...and will definitely perform for a very long time!
here are a few pics of semi-rigid manifold/wg braces that help these things live