That's a very sweeping statement! It all depends on the quality of the instrument. You can't expect a £20 one to work to the same tolerances as a £200 one. Maplin are selling one for £50 which has a stated accuracy of ±2%.Its just that they are not accurate enough to be useful for professional applications where its important to have an accurate measurement.
Hospitals now use IR thermometers instead of mercury ones; and I'm sure they wouldn't have changed if there was any doubt about their accuracy - after all a temperature of 101°F is OK but 103°F is getting dangerous.