Twitter thread on drawbacks of Mastodon's federated approach:
https://twitter.com/adrienneleigh/status/850243710973493248
TL;DR— Federation is great, but it kills privacy. If you want private posts, or want to delete posts, all you can do is request a deletion, but other Mastondon/GNUSocial instances don't have to honor that.
Moreover, it kills identity: anyway can be “@you” on some new Mastodon instance.
These problems are intrinsic to Federation, and will not necessarily be easy to fix.