Microfluidic reaction formats are of increasing importance to produce low molecular weight chemicals. In-situ product recovery in such systems benefits from intrinsic physical advantages of microfluidics, including increased mass and energy transfer and continuous processing. This opinion highlights the design space for microfluidic in-situ recovery of gaseous products with respect to rates of production and product recovery, as well as physical constraints limiting product titers. In a theoretical case study, O₂ produced by cyanobacteria in a microfluidic reactor is used to benchmark productivities through process simulations. In general, the productivity of microfluidic bioprocesses is confined by biocatalytic reaction rates, diffusion rates, and residence time. Key challenges for scale-up, and thus achieving high space-time yields, comprise biocatalyst formats, catalytic robustness, and specific turn-over rates, as well as numbering-up of microfluidic devices. Economically and physically accessible application areas may thus be limited to milligram or higher gram quantities of specialty products.