So running with 1 or 2 players makes most everything doable. But when a third player joins my squad even the mediums go to extremely difficult. The instant the ship takes a single bullet it's suddenly on fire with something broken, even if we had full health and shields. On foot isn't much better - seems like things hit a lot harder and also take longer to kill. I'm regularly dying to a plain raptor when I usually don't get bothered by them in solo or double. Feels very punishing to cross into the 3-man threshold.
I'm pretty clearly the worst player in my group, yet I can do hards solo without too much trouble. But we've stopped attempting moderates as 3.
I haven't tried 4 yet.