There's always a risk of things going awry with oil and it can be pretty messy when it goes wrong, but this isn't BP charging into the ocean depths and attacking whatever they feel like and to hell with the environment. This concept has been around and talked about for a very long time, if there were any fatal flaws in the proposal they would have surfaced by now.
The Keystone Pipeline is a massive pipeline that connects Canada to the US, which by default leaves many ways that it could break and spew oil all over the place. If the pipeline were to actually be constructed, it would be designed in such a way that if an accident were to happen, nobody would be hurt.
But digging it up and burning it creates a terrible mess, and because the pipeline is so huge, there's going to be a LOT of oil going through it. A lot of oil means a lot of smoke which is not safe to breathe. All it takes is one mistake for thousands of gallons to come out. There are better ways...Like not needing oil at all--electric cars, anyone?