Slack is a pretty good messenger for groups. It doesn't matter how big the groups are. It's possible to create different subgroups, so you won't get all the unnecessary information from other discussions.
The whole principle of Spaces is very easy. When you use Spaces it feels like community focused messenger, so like a mix of Google+ communities with Hangouts groups.
Spaces seems to be still in some kind of experimental / beta stage. Also the Play Store entry asks to say the develops what you're missing in the app.
But is Spaces really a possible enemy for Slack?
I don't think so. I think Google just tries to find new ways of how to enter the social media business. Btw. why don't I got an invitation to â€œWho's down by Googleâ€ although I registered more than half a year ago?!?
Decide yourself what you want to think about this and Spaces in General.