Space Marine 2's super soldiers are "almost invincible," so creating a genuinely difficult player experience was "quite the challenge"

gamesradar.com:

 Space Marine 2 is all about the power fantasy of controlling these hulking giants and mowing down hordes of Tyranids in front of you. However, balancing around such a powerful unit can be a tricky situation. Brand and creative manager Yann Francois shares how the team decided to balance around such absurd power levels to properly portray the sheer carnage Space Marines can unleash. 

In PC Gamer issue 400 Francois shares; "When you are reading about Space Marines, Ultramarines and Primaris - you discover that those are superhuman on a really crazy scale. They measure up to two metres or three metres [tall]. They have two or three hearts functioning. Because they are the only humans able to support such a heavy armor, and that scale of damage."

These giants are genetically perfect for fighting on the frontlines. However, that also means the team had to ensure that playing as one felt just as epic in scale as the lore makes them out to be. This turned out to be "quite a challenge for the team" and led to the design decision to limit multiplayer to three players. "That is what led to the choice of creating multiplayer of up to three players. Because many people asked 'why is it not up to four players?' They tried it, and they realized that it was too easy with four players. It was a massacre. So three was a good number, to find a good balance."

Of course, when you have overpowered units like the Space Marines, you need enemies to match that level. "It took quite a long time to find the right balance between being able to show hundreds of enemies coming at you, but Tyranids are not zombies. They have their own behavior, and their own way of moving, and their own way of attacking the player. Tyranids in Warhammer lore are a hivemind faction. And how do you find the perfect balance between a rushing enemy coming at you that has its own intelligence behind it?"

The mix of three-player co-op and a more intelligent enemy design helped developer Saber Interactive showcase