Einstein designed his 1916 general theory around the general principle of relativity (GPoR), by building it on a foundation supplied by his 1905 special theory of relativity (SR). But the universes of the 1905 and 1916 theories have fundamentally different and irreconcilable properties. SR math and the GPoR require different environments, and cannot coexist in the same larger structure.
A valid general theory cannot be based on SR, it has to be freestanding.
Under special relativity, the presence and motion of masses has zero effect on the shape of spacetime, or else the theory doesn;t work.
Under a valid general theory, the presence and motion of masses must warp spacetime, or else the theory doesn't work.
Supporting both conditions is geometrically impossible. GR 1916 is a geometrical theory that breaks the laws of geometry.
Gravitomagnetism (GM) describes the dragging effect matter has on light.
The general principle of relativity (GPoR) requires rotating and forcibly-accelerated bodies to drag light, which only works if simply-moving inertial bodies drag light too. Since SR only works if moving bodies do NOT drag light, a valid general theory cannot reduce to SR physics. The specification of Einstein's 1916 theory, which is defined as supporting both the SR math and the GPoR, is impossible to implement.
GR1916 is a classical theory that predicts singularities, which break classical theory.
Where classical theory smooths aa body's inertial mass to produce a gravitational field, smoothing momentum gives a gravitomagnetic field, whose existence then invalidates the SR-basis of the 1916 system.
Special relativity's relationships work perfectly for energy conservation, without taking into account the energy and momentum of the gravitational waves that must be emitted every time a system changes state.
Once we also take g-waves into account, Einstein's system gives more energy after a reaction than we put in.
The correspondence principle of Quantum Mechanics (QM) requires classical theory's smoothing effect to be analogous to quantum theory's Heisenberg blurring, again making gravitomagnetism fundamental.
Support for the classical analogue of QM's Hawking radiation requires gravitational horizons to be relative. Einstein's SR-based approach forces them to be absolute.
By default, a GR1916 universe is infinite, immortal, and static. Modern cosmology requires it to be 3-spherical and expanding.
Since cosmological horizons are relative but Einstein's gravitational horizons are absolute, retrofitting Hubble cosmology onto GR1916 combines two irreconcilably-different geometries and rule-sets.
A hybrid "GR1916 plus Hubble" doesn't work under basic topological transforms.