Yes I agree, any tubular manifold will crack eventually, it all depends on the properties of the material used.
The point I was trying to make was that nearly all types of stainless will start off life far more brittle than any type of mild steel, it has all that chromium and nickel in it to blame for that, so the properties inherent in a mild steel will take longer to reach that brittle stage when cracking begins.
Also Cast iron is difficult, but not impossible to weld/repair. If you use the correct stick and don't let it get to hot (you can even MIG it) then let it cool very, very slowly. I often put small cast objects/manifolds I've repaired in a tub and cover with warm silver sand or even in a warm oven to cool. No problem and no re cracking.
But still, like you say what can you do, the customer is always right and if that what he wants then thats he gets.