Overall they need to be separate because if I get a slack notification I know it's for work and discord it's not. But if Discord launched a thinly rebranded alternative for work I think it would be excellent.
Administration has been far easier. The bot ecosystem is thriving, both open and closed. Although, the 1 feature I truly miss has been exporting convos.
No doubt, if building community is on your mind, go with Discord.
Most of the communities I care about eventually end up splitting to 3-6 servers, with about 20 daily active channels in each of these. Discord just doesn't give you the peripheral vision to figure out what's important.
Slack has its own problems, but the emojis are free.
discord for fun.