The technical issues in particular are a valid point but also it is much easier for people who are looking for information if there is one specific place in which they can find it. Additionally, this is also connected to one of the reasons why I would feel slightly more comfortable if those communities were connected to an instance that is focused on Rust, had there been any issues and problems we are more likely to care and try to support each other as it should be in our mutual interests to see Rust related projects grow. I still do not get fully how fediverse works in details but to me losing even bits of information when an instance goes down would devastating. I already hated how subreddits going private blocked me from access to some information I needed or wanted to share with someone else. I think this kind of risk is unavoidable and rather to mitigate it I think trusting someone who would be willing to restore all the lost data makes more sense.

This all however boils down to the admin taking on himself hosting those communities. This would become an obligation for him and whomever would be joining him to help.

A side thought, especially in fediverse world I feel like it is all much more about trust and communication between individuals. With corporations there are at least some legal bindings keeping them in check.