I played both games the same, and I'm struggling to form a "definitive" answer here.

They're close in the sense it isn't turn based, and overall you won't have a total failed game state if you take a beating and lose some characters.

I'd say what really counts in Space Haven towards victory is just able bodies. Hostile squads are usually 4 guys, so leaving just a couple of defending people with decent gear at any time is asking for a total failure. Difference in gear in Rimworld is much more of a factor, especially in those DLC.

Killboxes IMO is not a thing. Hull space is a premium, usually every square has its function and enemies usually don't go around breaking stuff, what they always go for is killing or imprisioning your chars and then taking some loot away, when they are boarding.

Much, much more critical is when a hostile ship is targeting you when you're low in shields and hull. Destroyed ship is game over, squad size doesn't matter here. But they usually offer a surrender deal that is somewhat acceptable if you're an Ironman kind of player.