Small errors may throw the solution off the target. There are critical path based device sizing tools which are much faster in giving you an answer albeit it is usually wrong. Their usefulness drops off very fast as you move away from inverter chains. CellOpt is not a critical path optimizer. The effect of each path is weighted statistically, mimicking its actual realization under typical usage models.
Critical path based optimizers look at only a small part of the circuit at a time.
As they adjust some paths, they can not control their adverse effects on other parts of the circuit. It also needs to know what kind of loads the circuit will drive, and with what frequency. Not the clock frequency, but the usage frequency of the capacitive load. In the same way, it needs to know how fast the inputs to the circuit will switch and with what frequency. Load and slew frequency distributions can be extracted from existing designs.