yeah, i agress that it's extremely optimistic to use an oven! - i've seen board's that are probably 10*10 inches and they tend to be known as banana boards, you basically end up with unbalanced areas of copper, when the boards are heated they basically end up bending, most manufacturers when they manufacture solve this by not allowing the boards to bend whilst they cool down...but it can warp back under heat - including later processes like wave soldering.... Your pick and placer is going to have technology (electrical test, xray) that can verify that the all connections have been made......

..I wasn't just referring to cost of placement of BGA's, it's more a general cost, you've moved up a step in technology, you'd better hope that you've got the design right and that you haven't accidentally connected a gnd pin to vcc under that BGA otherwise you're in a whole heap of trouble...

Adrian