For me the lower center of gravity makes balancing harder than an upright although not enough to be serious problem. It's similar to the greater difficulty balancing a pencil on your finger tip compared to balancing a broomstick. The taller broomstick takes longer to fall off to one side and therefore gives you more time to reposition your hand under its center of gravity. With bikes the difference in heights isn't so dramatic and most people can adapt fairly quickly. But the old penny-farthings (or 'ordinaries') are very easy to balance even at low speeds - much easier than either regular diamond-frame bikes or recumbents.