• Bad instances could put false data into the chain, some vigilance for kicking and untrusting them would be needed
• If your account gets hijacked there isn't much you can do about it, no recovery (problem / risk in crypto that is well known) You are personally responsible for your account not the instance and lots of users and not smart enough for this.
• Typically to prevent abuse crypto chains specifically put in CPU/GPU compute load to prevent being able to rapidly abuse posting data to the network. This could be done client side in the browser but to some degree would be needed on the instances to prevent people just spinning up malicious instances without cost but if this overhead is limited to identity only IE user creation, sign in, update it would be manageable other than during rapit sign up events like Reddit going down, spikes in interest.

Nothing is free or perfect but the current system somehow didn't learn from decades of spam,bots, abuse in Email, IM and Web Forums. I don't even recall the last web forum phpBB, SMF, Invision, Discourse etc that don't OUT OF THE BOX give server admins a tool box of ant bot and spam / abuse clean up tools.