I never know whether I've just been fortunate in my career or what, but I've just never experienced anything approaching the kind of unkindness that I frequently see people say is common. I dunno, maybe it's just me, but I'm skeptical. I've always worked with professional colleagues who are trying to figure out how to do their best, and have never once seen people behaving in this childish way.

It really depends on the company/team. Certain companies (Amazon), use PR feedback stats as promotion metrics. They don't even consider the feedback quality, literally the stat 'average number of comments per PR' was one of the metrics used at my last team. The only 2 people promoted on this team during my tenure there got promoted based mostly on their loudness in PRs and team meetings and constant pushing for various (mostly counterproductive) 'process improvements' - these people didn't actually ship a single feature/project on the team. Very toxic place to work to say the least.

For every unkind reviewer I've encountered in my career, I've encountered at least one person who overreacts to review comments.

I also agree that both have been pretty rare for me.