I think the level of pixellation depends on what Mac you pair with what iPad. I'm probably the best-case scenario.
My setup is the 12" IPadPro with a late 2015 Retina iMac that has the quad-core 4Ghz i7, 4GB Radion video hardware, and 24GB of RAM.
I can paint in Photoshop, and get no pixellation, except when selections are visible (marching ants), or I'm using the Liquify tool for more than a few strokes (because it updates the entire image with each change).
I can sculpt in ZBrush with no pixellation, unless I'm rotating/scaling the entire model, and even then the pixellation resolves in under a second. This is under WiFi.
Sure, there's plenty of room for improvement in speed and other features, but the experience is functional enough that my Wacom Cintiq is sitting on the shelf. I find the lack of video and drawing resolution on the Cintiq more annoying than the pixellation and color drift in Astropad. At least when I draw a straight line it doesn't wobble as you get closer to the edges of screen.