It varies by sub, and by the purpose of the sub. Some are terrible, some are quite good.
For example, the mods of r/askhistorians are so strict that in most threads the majority of the comments are deleted. But they apply the rules consistently, and keep the sub tightly focused on questions answered by professional historians, in detail, with sources. If you have a historical question you'd like answered, it's an amazing resource.
AskHistorians is a great resource until you watch how topics in your area of expertise are answered and the kind of diversion from wide consensus that is tolerated with scant sourcing. It basically mirrors whats been happening to academia. As long as you can source a peer review on gay dogs in parks you'll be taken as credible.
I remember seeing posts there talking about how Ancient Sparta was never exceptional and actually lost most battles. As someone who has read all the primary source material on the Spartans...thats total crap.
I'd argue lately that most are terrible and a few are good. AskHistorians does that because they employ literl hundreds of mods. It's not a scalable solution.
For example, the mods of r/askhistorians are so strict that in most threads the majority of the comments are deleted. But they apply the rules consistently, and keep the sub tightly focused on questions answered by professional historians, in detail, with sources. If you have a historical question you'd like answered, it's an amazing resource.