The fundamental problem of Einstein’s field equation is as follows. At the left, you have the Einstein tensor which is pure geometry. At the right, you have the energy-momentum or – better – stress-energy or energy-density tensor, which is independent of the curvature field. Therefore, the latter tensor does not contain the field energy.
(see https://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html)
Therefore, Einstein’s theory had some success for explaining small deviations from special relativity in the solar system, but fails, for example, in explaining the velocity curves of galaxies.
Besides this, the approach (“ansatz”) of Einstein’s equation has consequences for the mathematics. The geometric quantities of the left hand side are equated to a physical definition on the right hand side which does not come from geometry so that the region of Riemann geometry is left. In doing so, one has to guarantee that no contradictions appear. However, when Riemann geometry is embedded into a “more complete” geometry, namely Cartan geometry, it comes out that Einstein’s approach leads to contradictions. His approach is only valid as a rough approximation to the higher-level geometry, where torsion is considered to be a minor disturbation.
Heim recognized this problem of Einstein’s equation without using formal arguments like Cartan geometry.
Heim published only few papers and most of his heritage is written on notice sheets. His computation of masses of elementary particles is not well documented, I never tried to understand this, although the results are convincing.
Gauge theory is not compatible with Cartan geometry. Gauge theory leans on a zero photon mass, which leads to a truncated form of electromagnetic waves (no longitudinal waves). However, longitudinal wave solutions are compatible with Maxwell’s equations. This is an argument that photn mass exists, and this falsifies gauge theories. Cartan theory delivers the Proca equation, which is the mathematical formulation that gauge invariance does not exist, and of course longitudinal waves are solutions of Cartan geometry. This is an argument for me that the extension of Riemann geometry by Cartan geometry is a good choice.