There have been people that have had them crack, and there are people that are successfully using them. The problem with them is not the weak material that causes them to crack... It cracks at the weld joints because of the stress.. I own one of these manifolds... I bolted my turbo to this manifold, and said to myself "NO SHIT These things crack! If i had this much weight hangin off my nutsack at 1300 degrees i'd crack too!" Its the stress of the weight that cracks them... That is why they need support...
The OEM Header uses supports because of its weight... not as much because of cracking issues, but more because if it didnt have that support, it could stretch itself away from the head and cause leaks.