Thanks for the reference to Whitecomb & Wilson, 1929. It led me to Medhat's 1985 "The digestion of dandelion pollen by adult worker honeybees
":

The gnawing and grinding  action of the mandibles does not  provide sufficient  mechanical  force to break the wall of the pollen  grains.

Fifteen hours after feeding, the rectum contained string-like fragments of partially digested pollen,  undigested pollen grains,  and masses of undigested pollen components. Our present observation shows that dandelion pollen is  not digested completely by honeybees. 

Such incomplete digestion of pollen grains and pollen grain contents undoubtedly will reduce the nutritional value of dandelion pollen to honey bees, and this aspect should not be overlooked in evaluation of the nutritional value of pollen to bees.

comments: 
It seems obvious that pollen is not an ideal food, being in a protective shell. It's almost like me swallowing whole nuts and relying on my gut to somehow get them opened up. 

Probably a properly designed non-pollen diet would be more easily digested than many types of pollen. Bearing in mind that pollen varies considerably in size and structure. 

notes: 
Size varies over a broad spectrum, ranging from a mean diameter of 5 µ in Myosotis to 177 µ in Mirabilis jalapa (AMELUNG, 1893) and greater than 200 µ in Cucurbita pepo.

The pollenkitt protects pollen from water loss, ultraviolet radiation, hydrolysis and exocellular enzymes; maintaining sporophytic proteins inside exine cavities.

PLB

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