[...] it sounds like you're arguing that the downtime is because of massive user registrations and not from an attack like they said themselves [...]
I have no idea where their downtime is from. If it is DOS-related, though, they would protect against it using a DDOS protection service like CloudFlare, which costs $$$
Lemmy.world hasn't done anything to suggest they would be a significant privacy and security risk to users, at least not yet
They have, though. The LemmyWorld admins doxxed a user who they believed (incorrectly) to be Hexbear admin CARCOSA@hexbear.net. Source: https://lemmy.dbzer0.com/comment/1754850
[...], pushing ads to federated servers or collecting and selling user data would absolutely change that I guarantee it.
We'll see, but the larger they grow, the more permanent they get, and ads only affects that so much.
I didn't and won't go as far to accuse the instance owners themselves of being behind the attack but I won't say it isn't a user from Hexbear because nether you or I can prove that it isn't.
Hexbear is not more suspect than other instances, and there is no reason to name-drop Hexbear, alone, in particular. If they're being DOSed, then whoever is responsible is most likely involved in a community that has a culture of DOSing in general, like a Chan, maybe the same one that has actively been responsible for vandalizing Lemmy instances.
However Meta decides to serve ads on Threads or Threads content. Whatever it takes, Threads will definitely do ads, as they are owned by an advertising company, but we don't really know how LemmyWorld will do ads until those ads are live.