It looks quite extensive, and should cover almost all practical cases. The main limitation I see is that it seems you are limited to picking the units to conve

It looks quite extensive, and should cover almost all practical cases. The main limitation I see is that it seems you are limited to picking the units to convert between from predefined lists. If someone gave you say a volume specified in nmi ft^2 for instance and you wanted to convert it m^3 you'd have to do nmi to m and ft^2 to m^2 and then multiply the results to get nmi ft^2 to m^3 because nmi ft^2 is not on the volume unit list. Those cases are probably pretty rare. It is not often that you run in to people who give you say a gasoline consumption rate of 23 picoacres or give you a cookie recipe that calls for 1.6 barn megaparsecs of vanilla. (BTW, those are 0.04 gal/mile and 1 tsp, respectively).