n810 said:
Takes more layers paint to cover the model and paint tends to rub or ship off more easily,
and generaly harder to paint.
And it has been scientifically proven, that models painted that way are much more prone to roll ones on their combat rolls and sixes on their characteristics tests. Also, they might cry at nights.
EDIT: I think the new GW paints are rather good. If you see a colour you fancy from the new range, give it a try! Also, some new paints have better coverage than the old ones, so if you have troubles with a particular paint, a solution might be found there. The new metallics are rather fantastic!