Harder difficulties always has a higher drop chance in every dungeon set but I don't believe the number of bosses is what affects it, just the difficulty. Because it's at the mercy of the RNG anyway though I ended up with more from adept than expert today but now I have an expert team sorted I'll be running that until I have one ADK skill maxxed.
Two of my five so far are from the bosses.
Two of my five so far are from the bosses.