Or they just play WoW but that is a completely different animal than a competitive MP game
WoW PvP has been trying for years and for the most part is competitive. I don't really know about wow's balance since I haven't played in forever. I'd say their SC2 balance is good now, wasn't always the case but a large part of that was due to them designing maps like a bunch of fucking retards, at least from my understanding. I wouldn't say HotS is particularily balanced, considering the small amount of available heroes and still having a good number of them considered fairly shit, however they have a decent thing going with the different maps having different heroes being good at them, making some not so great picks good on specific maps, so that's ok, and overall the game is young enough that I'd give it a pass. Hearthstone it's hard to say if it's balanced or not, I guess if we go with classes, then not really, if we go with cards, most of them are garbage but that's kinda standard for CCGs so it's alright kinda. Diablo 3 has no pvp, but is otherwise fairly balanced I think.
Overall I'd say their balancing is pretty solid, my initial comment wasn't about balance, it was more about their handling of competitive issues, the way they support the scene(or in this case, the way they don't) and such. For example, look at the difference between how Valve handled Dota2 release and how Blizzard handled HotS release. Now sure, dota would have been competitive from the start due to being a direct port of an already actively competitive game, but Valve spent a lot of money to make sure that'd be the case and that pros would switch over, they added a bunch of tools for competitive gaming support(ingame spectating, obs slots and caster slots, good spectator UI, replays, tournament tickets inside the game with rewards attached to get people interested etc) as well as simply running a big tournament to start everything off and make sure pros switch over and then a yearly large scale tournament.
Blizzard did like, almost nothing? They did an exhibition tournament with no prize pool(I think?) and only 4 newly formed teams. Understandably, there wasn't much of a competitive scene at that point. But even this year they still haven't announced shit as far as I know, they haven't been running large qualifiers or have announced a large prize pool to get teams to train and compete for it. The game's been out for almost a year, and it's still often around D3 viewers numbers on twitch, maybe a bit higher, but that's low as shit for a fairly new blizzard game, especially one in a genre that usually gets a fuckton of viewers.
The situation in HS is probably even worse, in large part due to the lack of Blizzard's support with competitive ingame tools. 90% of the tournaments are invitations only and always the same people, some of which aren't even actually great players, due to viewership and shit. You can't say "well I'm the best player so I should get invited", cause the ranking system is absolute garbage, and amateur tournaments are half scams half shitfest, with ridiculously low prize pools because they can't get much money out of it back. Blizzard hasn't been injecting some money to help those amateur tournaments, and they haven't been doing shit to the client to facilitate them.
Here I guess the issue is they didn't even consider HS would be successful or competitive, but that was like 2years ago when they released(or whenever that was), since then they barely have improved(they added a spectator mode at least, even though it's buggy as shit at times and a lot of tournaments ran the old double camera system instead). They're just letting it go on like this, and hope for the best. Kinda like the SC2 stuff, it feels they're just letting it slowly die.
So yeah I don't know about Overwatch being competitive/competing with CS:GO and/or CoD on consoles.