I did some measurements with my Keithley 236 (some measurements referenced to my R6581T) and it looks like it easily meets it’s accuracy specs for both current and voltage.
I measured and it has an input impedance of more than 100GOhms from 1 to 14.9V, above that the current increases dramatically, I assume this is a function of the protection devices.
As someone who was an original Kickstarter backer and has been using it since then in many professional projects, punching more I can say that I love the Joulscope, it has really changed the way we do low power development.