@jarkko It must depend a lot on the Discord server. I've only been on C64OS and Synthstrom Deluge Discords, and they're the friendliest places you can imagine.
@jarkko
No idea what else is on Discord, I'm only there because my two communities there aren't really reachable by any other means.
AIUI the various Discord servers are entirely isolated islands in terms of rules, accounts, policies and all. The only common thing is that they're hosted by Discord the company and that they're only usable by their Discord client (web or app). So it's not exactly an open thing.
As for the general atmosphere, I guess it just follows the common rule of forums and such: the smaller niche groups tend to be nice and friendly. When the number of people grows over a certain threshold, things start going downhill.