Generally, low rider racks keep the load centered on the steering axis and closer to the front axle. That mitigates the swaying effect of a cantilevered load. In theory, the bike would handle much the same as without a load, but would weigh more. In practice, there is some sway effect even with low rider racks.

The downside to low riders is the lack of ground clearance if you are riding off road or very near curbs. The higher riding racks often have a platform over the top of the wheel for additional load carrying, similar to the rear rack. I've tried it with the Surly Nice Rack and it isn't for me, but lots of people like the platform on the front.