The hateful comments are all passive aggression in my experience. I don't mean anything anti-semitic content or anything along those lines of hate - I just mean bad faith arguments, ad hominem attacks, etc.
As for non-mainstream, basically anything that the moderators don't agree with. Could be about technology or politics. Anything which dissents from the prevailing opinion of a comment thread is also at risk for being moderated. They don't moderate the discussion, they moderate the topic.
So a few passive-aggressive comments are "hateful"? I find this a really strange way to put things, to put it mildly, especially considering that overall, Lobsters does better than most of the internet which is often filled with much more direct content.
I rarely see content moderated (the mod log is 100% public). Lobsters is much more narrowly focused than HN, so off-topic stories tend to get removed. That's ... a feature, not an attempt to "censor dissent". Honestly, if you want to make these kind of strong accusations you should back them up.
The same applies to the "censorship by merging" claim from the parent comment. Reasonable people can disagree on how to best merge stories etc, and that's all fine, but claiming "mod censorship" is quite a strong accusation regarding the motives of the mods.
Also not sure what you mean with "protect opinions outside of the mainstream"? What kind of opinions do you mean? And how should they be protected?