I'll give you an example. My very first embark in DF was in a cold biome that froze over in the winter. Not knowing this or how to deal with it, once the water started to freeze and the ale supply was exhausted, my dwarves started to slowly die of dehydration. The last to go was a little girl. She died there alone and helpless, surrounded by goblins kept at bay by a single locked door.
DF is brutal.