The plus side of a full coverage block is that it cools the memory and a few other chips also, which you don't get with just a GPU block. So you may have to incorporate some air cooling and ram sinks.
The disadvantage, like Moto said, is its only good for that style of card, which can get expensive if you update cards alot, update the card, you need a new waterblock. So it's up to what you're willing to spend and what all you want to cool with water.