Passive regen is intended to be turned off, as part of the moderate difficulty increase.
This pack is about making combat non-trivial. Tougher enemies abound through Ender Zoo, and zombies spawn in greater numbers and are more persistent thanks to Corosus and their "Zombie Awareness".
Of note, to offset the difficulty of managing health, one should be aware of the means to replenish it beyond vanilla potions. Botania offers potions and baubles to aid in this, while Blood Magic does similar with potions of their own. Roots offers an early-game Health Poultice, which is a lifesaver.