While a bit more accurate than what was seen in WL, EE is of course not 100% realistic, by far ! Note that there were even more parameters (and thus a better accuracy) in early (pre-release) EE versions (see llrender/llatmosphere.* in LL's viewer sources), but these got deactivated ('gAtmosphere' stays always NULL in LL's viewer, so I simply got rid of the corresponding unused sources in mine), probably because it caused too large a slow down in the FPS rate...
As for the Sun disc diameter, it happens to be (almost) equal to the Moon's in RL (which gives us full Sun eclipses, sometimes), so the Sun disc is in no way "huge" in the EE renderer, but it was unrealistically small in WL and, in fact, the Sun disc was never properly rendered "near" (for elevations 45° or below) the horizon (it was also sensitive to the "WL sky details" setting, with huge distortions at low setting value, which is no more the case).