Okay, I compiled synfig from source using the git master branch to see if I could get it to work. Now it came with 4 brushes. A 0.4 mm pen, a 0.8 mm pen, and what I assume are two erasers. Using these tools caused exactly the same error described here: Brush tool acting weird
I also added the mypaint brush folder so I could use mypaint’s default brushes. None of these brushes worked! So I think the problem I was having before was that a) no brushes came with synfig installed from the Ubuntu repositories and b) brushes from mypaint don’t work with it. However, the default synfig brushes do work (in the sense that they draw… something) Except, they work poorly and draw disconnected lines. Possibly related, drawing with the brush tool is really laggy.