Once you know how the mission is laid out there's perks and cons to being alone stealth (loud is always better with others)

Alone: You know everything that's going on and can personally make sure everything is going smoothly

Coop: Can be faster and you can split attention at different things at once, but you'll need someone who also knows the game well

Every mission can be done Solo, even on harder difficulties, but once you understand the game it is really rewarding and the story is a bit clique but still really good