Maybe I am just am ignorant American, but Joe is Johnson at all comparable to Trump in terms of overthrowing democracy? I don't recall anyone storming Parliament or Boris claiming any elections were rigged.

Depends entirely on the subreddit, in my experience. Places like AskHistorians didn't even exist when the great Digg exodus occurred. My favorite sub was /r/cfb which also benefited greatly from the mainstream popularity.

Not coincidental that both of these are relatively strongly moderated compared to many of the biggest/default subs.