No doubt some nickel-iron type are not well suited for direct hammering. There are several factors to take into consideration... the crystal structure of the meteorite (from fine to coarse octahedrite, etc.), and metallurgy for example. The Gibeon Meteorite from Namibia, Africa is an example of a nickel-iron type that you can hammer directly without forging. Again, it goes back to the odds of the right type of meteorite falling in the right place at the right time.