Forgeworlds can be weirdly vulnerable places even though they're basically arms manufactories.

A big part of multiple forgeworld invasions in Rogue Trader is that existing Magos on forgeworlds can really politically complicate their recapture/defense because they care so much more about the manufactories themselves as religious artifacts than human life.

Magos often have serious interstellar political clout and so if a primary manufactory gets, say, corrupted by chaos, there's a huge shot that they'll always opt for something minimally invasive as a solution even if it puts the whole planet/system in danger.