Light (photons) reliable gauge?

Photons are not seen as beams. In fact, light streams are not reliable, among other characteristics, it shreds along its path, has gaps. Plasma may impede its progress. It is recorded as not maintaining an exact speed. A recorded measuring of light over some years back showed that light did not travel the exact same speed throughout. So what reliability in science is that over long distance?

In a reading from the 1969 "The American Handbook of Physics", has the 'adapted' value of the speed of light at 299, 792.456 in kilometers. Multiple Measures from 1945-1960, the readings found are ranged from 299,750 to 299,800 kms. Not very steady is it? Not "exactly" matching with Special Relativity.

Further, the universe is now found to have curvature in it. It is not linearly straight from all points, thereby light does not always follow a linearly straight line in this universe.

In contrast, neutrinos are found to travel largely unimpeded by anything and are believed to be everywhere, compared to light which may be influenced by geometry.