What makes a biocatalytic route scalable rather than elegant?

Biology gives wonderful selectivity, but the boring reactor details decide whether a CO2 route is more than a nice pathway diagram.

For enzymes and synthetic pathways, I would start with Schwander et al. (doi:10.1126/science.aah5237) and Liao et al. on enzymatic limits (doi:10.1016/j.biotechadv.2022.108024). For pilot-scale fermentation, Liew et al. is a useful case: doi:10.1038/s41587-021-01195-w.

Which bottleneck deserves the most attention in a first read: gas-liquid transfer, product titre, contamination, reducing-equivalent supply, or downstream separation?