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.

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.