I used to do this. It's not impossible I'll tell you that but here's a few things,
It takes a dickload of time to practice, you'll ruin your pens and tablets (I've gone through 3 in the last year, on my fourth now) and it's mostly about luck.
you sort of have to vibrate your hand, keeping it about a 1 mm off your tablet between taps. You can't do lengthy streams like the ones in most songs but you can do streams of about 10-11 no problem, after that you'll start to hit 100's, 50's then eventually miss.
My advice is to just go with keyboard & tablet.
