What happens if a server goes down or a server keeper decides they want to modify some post or comment? It doesn’t seem much different from what spez did, except now you put everything in the hand of another random person. Legal issues are potentially even worse in this case because there is no company behind the site to blame for their wrongdoings.
You put everything in the hands of many random people :). This is fundamentally no worse than the situation with Reddit.
You have to create tens of accounts if you want to post on other instances. Yeah, you might be able to subscribe to communities from one instance but you can’t post there, only on your own. Maybe I’m missing something here but it doesn’t seem like you can post to the communities of other instances.
That is not correct. I'm not sure what issue you're having, but if you give more details about what you've tried maybe we can help.
Alternatively: create the same community in your instance? That would just make useless duplications or even separate instances even more…
Again, no different from the current state of reddit, you just have many Gaming@whatever instances instead of /r/gaming, r/truegaming/, /r/really_true_gaming_for_real, etc.
There are absolutely rough edges here, but to be honest I think a lot of people either weren't around or don't remember the early days of Reddit. There were a lot of rough edges back then too.