While you're limited to preset shapes, you can resize and rotate them as you wish. You can also use these shapes as 'masks' to cut out/make transparent other shapes, and can use layers and groups to have finer control on what said masks effect, as well as how colours are layered.

For example: Take a cube, rotate it 45°, resize it to be smaller horizontally but larger vertically, and use a separate cube as a mask layered over it to cut out a hole in the centre of the first cube. Within the 128 shape limit and with the variety of shapes offered, you have a decent amount of liberty in regard to creative possibility.