With Hackmaster 5. The balance point of play is on health and equipment. This creates a long term dynamic instead of an encounter or "adventuring day" balancing act. Added with penetrating (exploding) dice and thresholds of pain (ToP) this makes even easy combats dangerous. So there is very little pressure on balancing a fight to make a challenge, every fight is dangerous. This is honestly the biggest flaw with GMing D&D 5e and PF2e, because there isn't really a longterm balance point. And giving players a little extra healing (bonus action healing potions) or a night of sleep makes it much harder to challenge them without a TPK. Which is a consequence of the mechanics fighting logic in the game.

Thanks to Hackmaster's longterm framework equipment can be very impactful on play encouraging exploration. And giving a powerful item doesn't create a future problem for me. I can just roll for items and it's fine. I also don't worry about mixed level parties, weak characters or broken abilities.

Hackmaster Monsters are well designed with lots of supporting information that help inform my choices and provide easy answers. Stuff like sleep cycles and spell components are clearly listed.

For WFRP and CoC, the d100 universal resolution system and simplicity of rules makes it very easy to arbitrate. Effectively there are few rules questions.

Cthulhu also follows a particular flow of dread, terror, gore/horror that push the game forward. But it does typically work best with one shots.