It's not a lot of times when Discord has a major outage. Most of the months have 100% uptime, and most of the outages are minor, and don't affect a lot of users. With self-hosted servers, it's unlikely you'll ever reach that high uptimes. Also, the Discord team can't guarantee the stability of the whole service if there are lurking self-hosted servers.
You will lose a lot of the versatility of having a cloud-hosted server. You won't be able to change the region in less than a second if something goes wrong, you'll have to fix it yourself.
Discord works on the rolling release model, which means they push a lot of code daily to make the service better. This means that, if you had a self-hosted servers, you'd have to upgrade the server pretty much daily, which will get annoying really quickly, and a lot of fragmentation between server versions will occur.
By self-hosting servers, there is the possibility of a security breach. Some unscrupulous server owners could get their users' IPs and sell them,