@mttaggart good list, but I don't think it's fair to Matrix today.
The CSAM attacks happened, but moderation tools have taken a big leap since. You can quite easily use a moderation bot that subscribes to public ban lists. This almost fully ends any generic spam.
Also you talk about matrix.org as "flagship server". Friends don't let friends use matrix.org. Use small/medium sized servers instead and you get better performance. Same applies to Mastodon - don't use mastodon.social, decentralize!
My main criticism would be the buggy mobile clients, and super slow migration from Element Classic to Element X. Element web/desktop are decent quality, but far from polished.
Oh and you don't even mention bridges. Other alternatives don't even try to support them. Bridges are a key feature.
The CSAM attacks happened, but moderation tools have taken a big leap since.
I would appreciate links to these new resources. Last I saw, Draupnir was still very much inadequate.
As for server choice, I think Mastodon itself is evidence of what a fussy server selection process can do to adoption.
This reply is written from the perspective of a computer enthusiast. Probably a volunteer sysadmin/self hoster—like me. We cannot be the target for a general alternative.
@mttaggart for example etke.cc offers a public draupnir instance: https://etke.cc/help/bots/draupnir/
That combined with good policy lists should be quite good against spam. Of course targeted attacks are harder to block.
Also I think I heard that in FOSDEM Element said that there will be some kind of server chooser at last.. IMO using matrix.org as default has been a bad choice. Of course a curated list of servers makes a handful of new problems.
Currently a list of open servers can be found at https://servers.joinmatrix.org/