Lakdawala, a noted London System champion, points out that 1.d4 g6 2.Nf3 Bg7 3.Bf4 is not a good move order as Black is ably to play ...e5 straightaway after ...Nd7 as he benefits from not having developed his king knight yet.
http://dev.jeremysilman.com/shop/pc/Modern-Defence-Move-by-Move-p3712.htm
I have heard that the London System doesn't work well against the Modern Defense. Does anyone know why this is the case?