Bruisers and Tanks are very similar, in fact Bruisers can even be tougher than Tanks.
What separates the two is how self-centered the hero is. Bruisers are very much self-centered heroes. When they stun someone, they do it with the intention of killing the target themselves.
When a Tank stuns someone, they do it with the intention of assisting an ally. Tanks are very much a supporting function.
This is the difference between Bruisers and Tanks.
Not really, I CC to kill people not to make scrubassassin 10 845 feel good about himself.
Yeah, because Moshpit does so much damage by itself.
You have just played against terrible mages, because Imperius gets blown by mages. He has no self sustain if he’s not in melee range
He is. With right talents of course. Remind to thou what Dehaka has most powerful self-sustain in the game.
I actually wonder who (reasonably) has the most single-target HPS in the game, I know Imperius with Brand of Solarion is a contender.
There’s also Butcher, Zul’jin, Tychus, Deckard, Leoric, Arthas, Mal’ganis, and of course Dehaka who has the most versatile healing in the game.
There are a few heroes that walk the line between Tank and Bruiser and can act as one or the other in a pinch, and I think Imperius can walk this line when necessary.
However, I don’t think he’s classified incorrectly. Tanks have to check a number of boxes in order to be effective in their role, and Imperius doesn’t check some that are fairly important. Some of those are:
Back in the day we also originally intended for Dehaka to be a main tank, and he had a similar issue in that he could reliably take a lot of damage, but his ability to CC was primarily tied to his Drag ability which wasn’t consistent enough to have him be a main tank due to its range, wind-up, and ability to only CC 1 target. I think Imperius falls into a similar category due to the nature of how Celestial Charge works. It’s not quite reliable enough to deter enemies from diving his team in most situations since it has a bit of a delay and hits a small area. His ability to survive damage is also tied heavily to his ability to use Basic Attacks against enemy targets while they have Valorous Brand, which is not as consistent of a way to survive compared to other tanks in our game.
I don’t think it’s impossible for him to be successful in the role, but I think there are good reasons why you don’t see him acting as a tank in most of his games.
Called it ! 
I can definitely see clearly now why Varian / Dehaka are considered Bruisers even though they are tougher than most tanks (lack of reliable, consistent CC.)
That said is the team considering another pass with hero roles? Such as reclassifying some heroes (from your description Cho would definitely fit better in the “Bruiser” category) and perhaps renaming the “Melee Assassin” role to something that fits heroes like Tracer and Genji?
Why is Varian still considered a main tank for matchmaking in Quick Match? I don’t feel like Taunt Varian has enough crowd control to keep up with a dedicated tank.
Efficiency of taunt Varian depends from both teams, so it’s hard to balance him in QM I guess.
I would argue the same about Imperius.
(As a side benefit, flipping Varian to actual Bruiser would get rid of those frustrating games where your ally goes Win Blades when you needed CC).
In pretty much every rank except for the highest dehaka and imperius can maintank quite effectively. When they’re good enough at landing their skillshots and the enemies are bad at dodging them they check both of these boxes.
But at end of the day they both make good-excellent bruisers and meh maintanks so they’re right where they ought to be.
It’s more about how easily the hero’s CC is countered vs. its power as a CC.
For example Xul’s “Bone Prison” is a very powerful CC … but it’s also very difficult to land, and easily countered.
On the flipside there is Johanna’s “Condemn”, which is fairly mediocre as a disable, but very reliable and easy to land.
(Reliable CC is more useful for allies, going back to the point that Tanks are a “Support” function.)
The best way to gauge this is to put tanks up against each other and see who wins “1st CC” most of the time. If you put Diablo, ETC, and Johanna up against Varian, Xul, and Imperius you’re going to get Diablo’s, ETC, and Johanna’s disables hitting first 80% of the time.
I’d say part of that is for QM matching; it’d be a lot harder to play around Cho’gal comps if the matches didn’t place him against an enemy tank.
Since a pocket healer usually follows the cho’gal, that’s half the team with him; bruisers are usually the off lane pick, and not in the main rotation so it skews the unorganized balance to essentially give one team a ‘typical’ rotation that needs some levels so their bruiser can get the talents they need for armor/sustain/cc to try to match that.
True, but then again it’s Cho going up against other Bruisers, which is honestly a fairly even match up.
For example if I were to take any Bruiser + Ranged Assassin vs Cho’gall, I’d find that they’d have fairly even success rates.
(For example, Dehaka + Lunara, or Greymane + Leoric, or Orphea + Artanis.)
I feel like this is #1 reason why last 3 tanks had problems. Or why Stitches fails.
Stitches is kind of a Bruiser by this standard, admittedly they’re trying to gauge strength * reliability.
A hook is a very unreliable CC, however, it’s far more effective than the more reliable CCs. Same thing with Garrosh. Garrosh has one of the most powerful CC combos in the game, but it’s also hard to pull off effectively.
Old Stitches with W build was one of the most fun offlaners in the game 
Many players know this but it just sound much better when it come out from the developer. 