How can we justify the assumption of symmetry used in the derivation of the Lorentz Transformation? It appears to be a form of naive impressionism or the belief that what is true for one is true for all. But it appears to fit the facts. The Michelson-Morley experiment gave a null result on the measurement for the velocity of the ether. The speed of light appears to be independent of the Earth's motion throughout the year at least for the value of the Earth's orbital velocity. Relativity has proven to be a useful tool for scientific research. But it seems to validate the simplistic worldview and the possibility of making a false assumption.
One could view Ockham's razor as a form of naive impressionism. But it is an economy measure. One has to seek a balance between ignoring the lack of evidence to the contrary and unnecessarily complicating an explanation of the facts. Making unjustified assumptions raises doubts.
The operational rule of the scientific community appears to be naive impressionism with doubts. So we are justified in saying, "Don't trust them." As a matter of expediency, however, this may be the best way of proceeding, but under the circumstances one needs to show that the use of Relativity is justified in a particular case and that the results are reasonable. The assumptions break down if there is an asymmetry in the point of view of the observers. This may be the explanation of the imaginary values in the transformation for velocities exceeding the speed of light. Strong gravitation may bias the transformation and Einstein attempted to address this in the General Theory of Relativity. Special Relativity may still be the best first approximation to the laws of the Universe.