I'll hazard a guess that since Reddit is at least vaguely self-selected topical, it's possible to evade a lot of the stuff any particular user doesn't like. I personally read some technical reddits, some palaeontology redits, and a few "metro" reddits that are near where I live. I've never seen the amount of puzzlingly irrelevant ads, bots, spam, racism and fascism I regularly saw on Twitter (I've deleted my account).

For Twitter, you get what the algorithm selects for you. Clearly that algorithm has some cut-outs for advertising, and particular accounts, and after that, I won't hazard a guess. But it's not topical the same way subreddits are.