If the connection isn't perfect, which can be really hard to determine when you have to check 400 connections all of which are sandwiched between two boards (so, X-ray), gotta bake it back into shape. If balls fail you need templates and fresh balls to re-ball the chips. Oh also routing sucks when you have to tie signal lines around inside a matrix of several hundred vias.
Add to that RF issues generated by that web of routes and you have a nice 5-6 months of debugging and hair pulling.