The accuracy depends on how accurate you build the frame, how accurate you can place the router on the sled and how good you get it calibrated. I’ve achieved < 4mm back in April 2017 and guess now < 2mm is achievable. The problem areas are the top of the sheet and the 2 down corners. In the centre I could cut to a 10th of a mm.
The repeatability is amazing. If you are 2mm of at one point of the sheet it will be the same on each pass. (until you change a setting or modify the hardware)