Function point analysis works well enough for measuring software productivity in most domains, provided that quality is held constant. Like other software productivity metrics it's only meaningful at the level of a complete product team and worse than useless for evaluating individual team members.

https://doi.ieeecomputersociety.org/10.1109/32.44364

So you can measure productivity with a reasonable level of accuracy and consistency, but then what? For most organizations it's not actionable so calculating that metric ends up as another form of waste.