Breaking down the problem it’s clear that there are issues with stealth.
But Stealth in itself is not broken or overpowered although I agree it is poorly implemented.
Stealth does not make Thieves disappear. In fact they will die in Stealth fairly often especially given how excessive CC and AoE is in GW2. With very few abilities actually needing targets and AI reacting instantly to stealth-mechanics it’s not nearly as powerful as it could be.
The problem is two-fold.
1. Stealth does too much.
-Stealth makes Thieves invisible.
-Stealth increases their movement-speed
-Stealth heals Thieves.
-Stealth can increase their Initiative regeneration.
-Stealth blinds people nearby.
-Stealth sets-up Backstabs, the biggest burst-attack in game.
-Stealth cleanses Conditions every 3 seconds.
Now depending on the build, most Thieves will have a number of these abilities tied to Stealth. It just makes Stealth incredibly powerful and stealthing as frequently as possible becomes necessary for success.
2. Thieves rely on Stealth.
Given all the benefits of Stealth it is no surprise most popular Thief-builds somehow involve the frequent use of Stealth. Evade-spam being one of the few exceptions to the rule. That’s because Thieves are completely nonviable without.
Take Adrenaline from a Warrior and he can still function reasonably well. But take Stealth from a Thief and you are left with a squishy melee-class with no redeemable qualities.
I’d actually like to see a good and more well-rounded Thief build that didn’t just rely on a single-gimmick or synergy-abuse. But given the mechanics ANet has put in place I don’t see that happening anytime soon.
PS: I also don’t believe the currently popular Dagger/Pistol build was ever purposefully designed with the current play-style in mind. It just evolved as players were forced to adapt to an increasingly CC- and AoE spaming environment.
Because let’s face it, creating a combo field and leaping through it 2-3 times is and extremely convoluted stealth mechanic.