Comment by rollcat

Remember that factorio simulates every inserter swing, the exact position of each item on a sushi belt, every dip in the power grid, every biter egg about to hatch.

Some designs in 100% runs use sushi (e.g. red circuits, as they take a lot of space/time but few resources, and must initially be built by hand). You can estimate the rates, but you won't be able to find a bottleneck.

Inserter timing often comes up in plastic builds - on paper it's easy to build the exact amount of chem plants to saturate a blue belt, but if you ignore inserter timing you'll be leaving tiny gaps.

Also remember spoilage is now a thing (,: as well as loops, probabilistic recipes, thruster efficiency, probably some more.

Only Factorio can simulate Factorio. The best way to automatically find bottlenecks is probably with a mod.