We don't quote numbers since it has so much to do with how the data is collected, but the elevation is generally within 1% for relative measurements with well collected data. Since the Inspire relies on GPS and barometric altitude measurements they can be off by quite a bit. If you upload a ground reference image (image taken at the point of takeoff) with your imagery we use STRM satellite data to give an accurate starting point. It can still be off by a few meters.
The short answer: in a relative sense it is quite accurate, with respect to sea level it will depend on a few external factors.