So, Contrasts in the hands of non-invitational, non-professional and semi-professional painters...
Pros
It's fast.
Con(trast)s
Contrast paints are finnicky to use.
Similar to washes, it's better to go with less than you think you need, otherwise pooling happens. Which is in direct opposition to 'One Thick Coat'.
It's better with two thin coats - one coat is likely to be inconsistent and leave splotches. Doing two coats means no time saved. Especially as a Contrast basecoat takes longer to dry than a normal Base...Uh...Coat.
It's very, very difficult to fix mistakes.
It makes your model look washed (like Glaze-painting does). So it looks even better with traditional edge highlighting or drybrushing afterwards, which is how you'd normally paint a model after washing it anyway. So it's not actually faster if you actually want your models to look good.
It doesn't taste nice - for those who lick their brushes
So...Trade quality for speed. Somehow I'm pretty sure I saw that coming.