In my past session, I found that mooks are stronger than I anticipated. I designed a heavily-mook-based encounter with a boss summoning a pack of mooks every other round. However, PCs were almost shredded and I had to stop the boss to summon additional mooks.
Here is my idea:
In my calculation, one mook is approximately equivalent to a 3-lower-level monster. A 3-lower-level monster who has half hp in exchange of boost in attack (+2~3), AC (+2~3), and damage (x 1.25, which is about x 0.7 for same-level monster). However I can see that a monster becomes far weaker than game difficulty math (two times stronger for every two level) when the level gap between the monster and PCs is widened. Therefore, I have no problem with the fact that five 3-lower-level monster is equivalent with one same-level monster in difficulty.
The problem I see is their pure strength. Of course, PCs should have some countermeasure to wipe the legion of mooks. However, not every class have such AoE power and even the classes with AoE power have limited use. If PCs cannot wipe the mooks before they get their turn, a pack of mooks attack with damage that is 3.5 times of their equivalent single monster. If the mook attack is concentrated into one PC, then it can cause a insta-kill for a squishy PC. Which players does not like at all. And that is just about one pack of mooks. four packs of mooks are properly difficult for four PC party in theory. I suppose that would make the game a hell in reality.
Am I interpreting some rules wrong? I want to hear how you deal with mooks in your game design.