I don't really know how this all works, but can't I (a hypothetical human) just create a user account on each disconnected sub-graph? Why is this significant?
Imagine your mom emails you and asks why she can't access her favourite subreddit anymore and you have to explain she now has to make 2 different accounts to access different subreddits. I mean 'instances'. I just picture my own trying to wrap her head around "why". It sounds like a minor hurdle to our demographic but most people are super lazy and bad at juggling online authentications. I would never recommend Masto…
But you also have the option of using an instance that didn't ban the community you're interested in, which you don't with reddit.