For which stellar phase is the mass-luminosity relation specifically strong and reliable for calculation?

Stars currently fusing hydrogen in their cores (main sequence stars).

The mass-luminosity relationship, defined by $L \propto M^\alpha$, is noted as being specifically strong and reliable only when applied to stars that are actively fusing hydrogen in their cores. These are the stars situated on the main sequence of the Hertzsprung-Russell diagram. During this longest phase of existence, the star maintains a stable hydrostatic equilibrium where fusion pressure balances gravity, keeping the relationship tightly defined. Once a star exhausts this core fuel and evolves into a red giant, structural changes cause luminosity to decouple from its original mass in a predictable manner, rendering the simple power law invalid for that evolutionary stage.