Caffeine is distributed throughout the coffee bean on a microscopic, biological level. Fentanyl is distributed in pills by a Mexican with 5 lbs of corn flour, a stir stick, and a pill press.
The issue is that a tiny clump or the stuff can stick together and then you get pills with 10x the intended dose. The safe way to do it--and the way that real chemists create dilutions--is to dissolve it in an excess of liquid with whatever you're trying to mix with it and then evaporate it under vacuum. Unfortunately, fentanyl is insoluble in water, so you'd need an organic solvent, and who the fuck has time for that?