4 Players on a difficulty 3 Cargo mission, here was the scenario we ran into:
We are in the last sector of the mission with low ship health and not enough materia to craft a new core repair kit.
As we engage the final area, we determine that keeping the ship alive will most likely be impossible with the number of enemies we need to fight.
We decide to keep the pilot and the gunner on the ship and engage with the enemy ships while the other two crew head to the mission objective to find materia or kill the jammer.
When the ground team got to the door, we couldn't do anything because it required all 4 players. This really kills the momentum and immersion of the game because now instead of gambling on a split team, we just sat around while the ship team tried to fight the rest of the enemy ships.
Basically, locking progress by needing a certain number of players just to activate something or be in the same area is, in my opinion, a bad mechanic.