[Detective100]

Since Cotillion is being coy about what Muscle Armor does, I'll explain.

Muscle Armor is an aura for unarmed/armed users that gives good stats (don't remember the specifics), with its main draw being its Q ability.
It is a temporary CC immunity to everything. Can't be rooted, stunned, blinded... Not even knockback works, so moves like javelins and dragging beams won't stop them.
The only way to properly avoid a melee user with Muscle Armor's Q ability up is either by using speed boosts or by turning on invulnerability. Otherwise? They are going to go in, and they'll hit hard.

It is a strong ability in an aura that makes it worthy as a hidden ability. Can literally turn battles around and make certain match ups turn into the armed/unarmed fighter's favor.

Assuming the wiki isn't lying, it gives +30 power and +10 vit. That's worse (in my opinion) than the public Holy aura, which gives +5% DR and +30 power, and way worse stat-wise than a lot of other auras such as Assassin's Shroud, the crystal/cosmic auras, and Chronos.

The saving grace is the active, which is definitely strong from what you've described. But, in this age of eternal slows, knockbacks, and extreme bursts that the current meta has, you're at most going to be able to use it twice per fight. 

Given the above and the fact that just about everyone has an invuln or speedboost now, is it really that oppressive?

30 power and 10 vit is nothing to sneeze at, considering it can be used with any armed build, and the active makes it much more useful than the public holy aura that has nothing but stats.

How much opressive the ability is depends on its duration, truthfully. If I remember, it lasts around 10 whole seconds. Enough to wait out an invul move or a speedboost (if you don't have a speedboost of your own). You may only be able to use it twice per fight, but that also means twice per fight you'll have a much easier time dishing out your high damage move on the opponent, which is where armed builds tend to excel.
If I am mistaken? People are welcome to correct me.