So,if users are very huge in single instance(mastodon main instance is the example) and the moderation is not likeable,there will be problems.
Also federation like this is new to people.
Will it work out? Currently, not likely.
Also,it can't be claimed immune. Bugs might be present.
A quick eyeball of the English-language instances listed on instances.social, sorted by # of users, would suggest that this is true. There are a couple very large general-purpose instances, but after that it's largely much smaller topical communities.
My hypothesis would be that, if one tracked these numbers over time, it would turn out that the larger ones aren't just bigger, they also grow at a faster rate. So, if Mastodon were to grow to the scale of one of the major social networks, we'd also find that the users were mostly all clumped into a single instance that's roughly the size (and level of nastiness) of one of the major social networks.
At which point, from a social impact perspective, the fact that Mastodon is, from a technical standpoint, open and federated, would be beside the point.
Furthermore, I think that small instances are just as important as the big ones. If a small group of people has a shared interest and they want to run an instance in a specific way that's possible to do with Mastodon.
There are tons of small Mastodon communities for all kinds of niche interests right now, and their users can make up their own rules for how they interact, what they allow, and so on. This is an incredibly important aspect of the federated model. There isn't one set of rules that applies to everybody the way there is with a centralized platform.