Just on this, most Discord mentions I've come across on HN describe how the user experience is what separates it from the competition and I'd agree.
The networking effect across communities doesn't seem anywhere near as strong as traditional social media where one might argue it's what keeps users retained. Few I've observed in the years I've been on Discord seem to care or recognize who someone is across 'servers' (Discord meaning), which is made less obvious by the encouraged feature of changing handles/avatars across communities to maintain quasi-distinct identities (even across related gaming chat). It's also not uncommon for some users block all PMs and friend requests from communities merely due to spam issues.
The communities using it that I've experienced are often doing so since it's the most convenient service for their purposes. Users can join with invites without signing up (a la IRC, traditionally), supports inline attachments (acting as an all-in-one image/video/file host), includes persistent, easily searchable message logs and just as importantly is all free.
A decent number of the communities I've encountered (ranging from software, subreddit/forum chat, game modding, media creation) could migrate elsewhere if there was an alternative suitable for their own needs. A number of them have migrated from other platforms before and one of the gaming-related ones even discussed potential future alternatives. If anything the self-hosted aspect would be the main drawback to a Discord-equivalent alternative (ie: the costs, maintenance).