It creates un-needed back-faces and polygons on export, and it doesn't give you much control on scale and normals on export.
The only thing it creates extras of are verticies, which is as simple as W, 6 in edit mode. Ive used Sketch for a long time now and have not had any issues with it that I couldnt easily resolve.
The backfaces, actually, are a result of carelessness. Lots of people color their model before its done; by doing so, you cannot tell whether the faces are facing the correct direction in sketchup. The front of a face in sketchup is a different color than the back, and you can reverse them by just selecting it, right click, and choose "reverse faces".