The current engineering productivity systems most actively utilized in the industry are based on engineering deliverables, e.g., isometrics, P&IDs, etc. This has been driven by the pre-modeling era of project document generation.
RT-156 found that with the widely adopted 3D modeling execution, a better representation of engineering progress can be correlated with the engineering units, e.g., lineal feet of pipe, cubic yards of concrete, tons of steel, etc. (RR156-11, p. 87)