Even in large niche subreddits like /r/coffee, /r/homebrewing, and /r/skincareaddiction which are all over 1m followers, there's pretty good self-moderation and calling out of astroturfing. /r/skincareaddiction might be the most susceptible to it since there are just so many products and skincare is pretty specific to the person, but I still see folks calling out shill posts and comments.