That is the common misconception about the ebay manifolds.
Even the supra guys had a few rumours floating around saying the ebay mani’s cracked. However, not one person to date has shown pictures of a cracked manifold, and it has become a much cheaper alternative then buying them from turbo shops.
Furthermore, the only real thing I would worry about is all the slag from the welds on these manifolds that could cause some damage to the turbine wheel. Just be sure to clean it out thoroughly before you install it.