That's different. In a wc setting big teams will be much more ruthless. See Spain. They are shit in friendlies but on the big stage they have been great (bar the confed final but even then Brazil were ruthless)
They might or they might not. Italy didn't make it out of their group in 2010 and neither did France. Spain made hard work of their group too. Brazil haven't had a great time in the past two world cups.
On paper, yeah, the defence might get mullered. It doesn't always work like that though. It's like the knobjockeys on radio talking up Arsenal's chances before the United game: "Their midfield is far superior to United's therefore they are going to dominate and come away with a convincing win".