I only visit like 2 reddits for hearthstone and they have typically been fine. I have gone to other reddits though like ranime when I was a panel member or epic seven and kings raid when I played them.
Obviously hearthstone and battlegrounds reddits are large. But on the bg one at least the vibe is that people are pretty helpful and positive towards things. Except beasts. The reddit hates them and one of the first things a new user should know is not to post how you won with one unless it's a weird build.
For something like epic seven which I played in it's prime the reddit has basically always been new players asking for help and fan art.
A lot of it does come down to the mods and rules on a reddit etc. Social boards like b8 are a pretty special card for gamefaqs and the wide breadth of insane and meme posts we have would not fly on a lot of reddits.