Those unit conversion libraries look awesome! Way better than my pile of hacks. I only added the units I happened to use in recipes (surprisingly few). The most hacky thing is probably all the cooking-specific ways of saying "thing" (clove, pack, fillet, sprig, scoop, etc.).
The most interesting problem I ran into with unit conversions was actually around the user experience: when did I want units converted at all? Right now, the app simply always does it before rendering anything. This means converting to the smallest unit of that type, and then picking the largest reasonable unit to display. This can lead to annoying instances where you put an ingredient in one unit, but another is displayed. I also went a little overboard on the fractions, so you see things like "2/5 tsp" which is just absurd.