Depends on your definition of "accurate". The 744F's W&B system has a 2% tolerance, so MTOW could show 8T plus or minus (in practise I've found it more accurate than that). Was involved in discussions with Airbus for the A380F; we asked for a 1% tolerance, they didn't think that was possible at a reasonable cost.
The system uses sensors in the gear to work calculate the weight and balance. Obviously a flat ramp is required to get an accurate reading.
As an aside, if you ever see a large unexplained error, trying pushing the a/c back and pulling it forward again. Worked for me a couple of times.