Pulling my hair out now...

I'll try to give as much history as possible: I've had Krita installed on Ubuntu 16.04 and it worked perfectly. Then, my mobo died. I went from an AMD to Intel core i5, and also installed Ubuntu onto a new SSD. So I have 2 installations of Ubuntu. The issue is that on the newer install, the freehand brush tool does not work for my pen display (Huion GT-190). Using my mouse button works. Every other tool works on my pen display. Just not brush tool. This is with the latest, brand new app image for Krita, default settings/configurations.

I want to rule out that it's an xinput or pen display problem. I use 3D Coat just fine with the pen, and like I said, every other tool in Krita responds.

Please help before I lose all my beautiful hair.

Check whether xinput or libinput is used to communicate with the wacom driver. One or the other doesn't work for some weird reason, for some people.

Update:

It seemed like running Krita from the terminal fixed it, but apparently it was short-lived.

I'm having a very similar problem. I have Ubuntu 16.04, with the AppImages Krita, and a brand new Huion H420.
Using the pen on the canvas is completely fruitless, but the mouse works fine.

EDIT: Fixed by following the instructions from the second answer on this question, BUT, since you're running 16.04, you only need to set up the "52-tablet.conf" file, and maybe do the bit about mapping it, though I haven't tried that yet. It says it's for Huion H610, but it worked for my H420, and it seems generic enough to work with any.

https://askubuntu.com/questions/500141/huion-h610-tablet