Far More Dangerous Mobs makes common hostile mobs notice you from farther away and move a little faster, so they feel more threatening without changing their health or damage.
Zombies, husks, drowned, and zombie villagers spot you from farther away and run. Skeletons, strays, and bogged do the same and can shoot from farther out. Creepers also detect you sooner and move faster. Spiders and cave spiders have increased detection range but still only become hostile in the dark.
The pack only touches those mobs. Everything else stays vanilla. It’s a behavior pack