Except that on most tablets and with most "styli" your penmanship does not really work. Most calligraphy effects depend on two elements: how much pressure you apply, and how fast you move on paper (this is especially true for Eastern calligraphy, but I suppose it holds for western styles, too)... pressure requires an expensive stylus, and a tablet that can interact with it (usually by Bluetooth) so it requires quite an investment (a cheap fountain pen will cost you 1/20th of it probably, a good roller gel pen would cost even less).

About the speed, I suppose it's less of a problem than pressure, but low-cost tablets would probably struggle with that, too.