IME an error of 30 degrees is not unusual if there is a coast nearby, your track is well away from being perpendicular to the coast (so the distortion is nicely assymetrical) and you are just the right distance away.
For example fly towards the Lydd or Shoreham NDBs, at say 45 degrees to the coastline, on a precise GPS track (autopilot coupled), and see where the ADF is pointing as you are in the 2-5D range. Closer in it is OK and further out it is also OK.
This is why most people fly NDB approaches using a GPS, checking the ADF at the FAF or whatever but ignoring it afterwards.
Over flat open country, say middle of N France, I find NDBs are very accurate - within 1-2 degrees.
As Whopity says they are also super accurate when crossing bang over the top. In fact I would say that is their only accurate aspect in terms of practical flying, because one doesn't need them for enroute nav
