It is true that the gargantua only sparringly returns units to the infirmery. I notice that having 3 different types of golems, e.g darkened, eldridge and standard golem are causing that you'd revive a few barbars for the golem type that are the least represented .

 E.g suppose 100 standard golems fall, and  10 darkened ones, you'd get only 3 darkened barbarians, despite you lost over 110 units. The fact those barbarians you get from the infirmery are  darkened clearly shows that the gargantua only covers up the 10 darkened golems and not the others, and surely does not cover up for those golem units you lost the most. I admit its a disappointment , I have no clue if this is a bug or intended . Only a developper can answer on that one. 

The same happens when fighting battleground , but there most players, as I do, attack with units of 1 kind and you won't have that dissapointing outcome.

Yes, you need to be very careful to choose your unit to send with gargantua