Scholar is the easiest healer, white mage is just annoying. AST is probably the hardest one but that’s because it requires the ability to split concentration between cards, healing and targeting the correct teammate/enemy with heals/aoe. They aren’t really hard in absolute terms though unless your teammates are collecting damage deliberately. All tanks play exactly the same in dungeons: you go in, gather mobs, mitigate, take a bit of damage and watch the healer panic and completely waste your self-healing ability.
To answer the title of the thread, because below extremes the difficulty is entirely theoretical. When was the last time you wiped on a dungeon or even normal trial?