I don’t think user voting in of itself is a problem. It’s the consequences of large negative voting that causes the real problems. In Reddit, a single unpopular comment on a popular subreddit could send a casual Redditor into negative karma which effectively shadowbans them from Reddit. As a result, you see people deleting their comments to stop the bleeding. Controversial opinions are punished severely.

Personally, I found that karma led to self-censorship of any idea that remotely deviated from the group consensus.

I agree; however, the second point I don’t see as Signal specific. In any service, how do you verify that a server is running unmodified open source code? For the vast majority of people, they are also depending upon the client being unmodified.

Yes, it’s not ideal. Decentralized key distribution seems to be a intractable problem for mass adoption.

What security issues does Signal have?

I 100% agree that Apple wrong for not supporting RCS (and I am an iPhone user). However, I personally choose to only use E2E for my communication be that iMessage or Signal.

RCS doesn’t have a cross-platform end-to-end encryption solution. For cross-platform communication, cross-platform 3rd party solutions such as Signal work well.