ECO is very peaceful. As of v0.5.4 there is no player damage or any hostile interactions. That being said the point of ECO at the moment is to prevent an asteroid that is slowly making its way towards your world. After 30 days (default) if you have not developed the sufficient technology to stop the asteroid everything will be destroyed. So stress wise I guess there is that sort of race against the clock element, but if you have a good group of people all working together I imagine it wouldn't be that hard. Unfortunately I haven't had the chance to play with a group yet but I imagine half of the experience would come from emergent gameplay with others.
Hope this helps!