They can be set very accurate, but keep in mind, there's always the variability of tyres and road conditions if a wheel driven pick-up of what ever type is used. While a GPS pick up can be more accurate, these too will be out if there's overhead obstructions, like building or trees, even on grades, if the unit can 'see' enough satellites, it will under read. They are also utterly useless in a tunnel, which is why they aren't legal in all countries. Personally, while it has the above mentioned short comings, I prefer a wheel driven pick-up (via the gearbox, from the normal speedo drive). If it was good enough and legal for the car as produced, then it's much less likely to get you in trouble.
While I haven't tried that pick-up of Poolguy's the issue I have found over the years with such a pick-up is there doesn't seem to be anything decent and medium term (let alone long term) reliable for the Mini. I've give up in recent years and made my own. It reads down to 0.25 kph and is super accurate & so far, 100% reliable.
Another great feature of the electronic speedos is it's very easy to change their calibration say for a change of tyres, wheel sizes or final drive ratios.