Hello Jose,
We can only know our measurements, observations and understanding of "what is". We can have an understanding of the Universe. But not understand the Universe. This is self-evident. No less evident than not truly knowing another human being. Such understanding is not limited to mathematical equations, however. Though these at this time provide the most objective reasoning and self-consistency.
But we should not confuse our understanding with "what is" the Universe. And yes, any claims that we know "what is" the Universe are metaphysical and lead to intellectual "religious wars". You write, "we cannot expect to understand the real nature of physical reality. Am I right?". That is my view. We can't in essence.
Planck's Law I have shown is a mathematical identity. As any mathematical result, it can be derived using various methods. Planck originally derived it using what he thought was a mathematical trick. Einstein latter proposed the physical existence of energy quanta in his derivation of this Physical Law. But Planck's Law is a mathematical truism and not a physical law that depends on the assumption of the physical existence of energy quanta. The issue here is not whether we can derive Planck's Law "with or with-out quanta". But rather between mathematical truths and physical laws. I am arguing Physics can and should be based on mathematical identities.
There are many interesting results that come out of this. Please read my Chapter for details. "The Thermodynamics in Planck's Law".
But let me highlight two here.
The Second Law of Thermodynamics: all physical events require some positive duration of time to occur
"If the speed of light is a constant, then light is a wave"
Constantinos