I do not believe thimbles are a good idea to use as the sole brake on a cart. I have a sulky harness that came with thimbles. This method appears to use the fore of the horses for braking. Not a good idea on inclines. It is fine for level ground and help slowing momentum, but I do not believe it is suitable for regular harness work. With breeching, the horse uses his hindquarters, which are stronger and better able to support the weight. When I watch my horse back up, even on a level surface, using the thimbles, it is obviously not as easy for him or as good for his body as when he backs with breeching. The saddle pushes forward on him backing up with the thimbles. With breeching, the saddle stays centered on his body.
I know there are folks that drive without breeching, but for my terrain breeching is necessary.