The amount you heal depends not only on the healing skill or potion, but also on each hero's recovery stat.
Some monsters have a skill that flips your REC to negative so you can't heal at all!
Similarly, amount of shield your shield skill can generate depends not only on the skill, but also on each hero's defense stat. Some monsters and Titans have a skill that can flip your DEF to negative, reducing your shield to zero!
When the monster flips your DEF to negative it will do more damage the more DEF you had, so using heroes with strong health and attack can be more effective than heroes with lots of DEF or DEF runes.