Leaving outh the part holding the device so you only have the desk for the textblade, I see two possible issues. The length across you lap is quite similar to a thin board I've used and, while it works, I'd get more stability as I type if it was longer.
The other is about the depth. My board is almost twice as deep yet stability seems even a bigger deal in that direction.
I don't mean the board falls off or something, but the act of typing on something that small causes "bouncing" of the desk every time you hit it.
Using my roughly 9x12 inch plexiglass sheet provides good stability. In fact, it also lets me rest my wrists on desk which probably makes it even more stable.
I can't tell from your picture if the part that holds the tablet would also increase stability in the depth or not.
But it is a brilliant idea for many situations. I have sometimes wondered about creating something that would have rigid strips slide out to give more stability. Wouldn't have to be real long to do that.