r/miata - Useless for the entire subgroup that takes their car to the track, and will very likely still give advice, it'll just be bad or outright dangerous.
r/cigars - Totally jump on bandwagons, and generally are "new" to the world of cigars.
To highlight another point, a lot of more niche subs largely cater to those new to the niche. Those are the same people commenting and giving others advice, whether they really have the knowledge to or not. This mechanism over a few months seems to turn everything into an echo chamber, and you end up with new people jumping onto a bandwagon without enough info.
Same goes for Amazon reviews, to a large degree - That $50 set of sockets might be great for someone who wants to change their oil filter, but will literally shatter for someone doing more involved work.
I think this is where the traditional forums are much better. I want my info from "serious" people who hang around the community for years, not from someone who came into it a few weeks ago, and really just want some upvotes/points/whatever.