On Surface ARM the stylus pressure is not recognized, whichever combination of settings (including enabling and disabling Wacom support, even if the stylus is the original Microsoft). It works perfectly with Photoshop and all the other applications.

There is no way to get the stylus pressure working on Illustrator. I tried going through the settings of the brush, enabling/disabling Wacom support, trying all the options for windows ink, and also switchin to the touch setup.

One interesting thing, once I saw it working for the first stroke, but is unclear why, and I was never able to reproduce the situation. Furthermore, by trying changing the brushes, time to time the brush does not write at all, and only after changing some settings (even getting back to the very same one) it resumes writing.

In the very same machine and system, Photoshop works perfectly fine.