Every place has their upsides and downsides imho.
r/Overwatch is pretty nice place, but front page is full of highlights, and balance discussion threads rarely get upvotes. But i guarantee that pretty much all the unconstructive and whiney posts will be downvoted or removed there.
As for competitive subreddit, i personally dislike it because their discussion tends to be extremely one-sided. But they are often talking about balance.
These forums are, in my opinion, often the best place to discuss about game because lack of downvotes means that discussions usually has posts from all different viewpoints. Downside is that you often need to see tons of unconstructive and whiney posts here…
I’d say it’s worth trying every place and see which one you like most.