It's kinda like, no one else has so much in terms of being a solo character. Everyone has something that benefits their team, but it comes with a weakness. For example Bloodhounds Q marks enemies but reveals his location simply due to the VFX. Lifeline can heal her team, but her drone is stationary and requires you to pretty much sit still. Gibralter has his dome which gives him a myriad of benefits but his location is super duper revealed, and only works inside of the dome, and the dome itself can be easily used against you. Innate difficulties you'd call them.
TL:DR, the reason many people feel cheated when playing against Wraith, especially in terms of size, ability strengths vs. innate difficulties, and overall mobility, is because she was designed around going alone and has the kit to back it up, while every other legend has innate penalties to balance out their overall utility that they bring to the team. With good comms she has two get out of jail free cards, one for her and one for her team, as well as the most beneficial passive in the entire game as information is often the key to winning an engagement, rather than how well you shoot. Wraith doesn't have any innate drawbacks to work around aside from improving your aim, movement, and general game knowledge (all of which are things you need on every character), thus creating a highly rewarding Legend that's easy to pick up, and easy to master.
A lot of characters save for Wraith and Octane have innate size penalties too. If they're not tall, they're wide. If they're wide, they're usually not tall. Gibralter and Caustic are both mostly tall and wide, and got a 15% damage reduction because of it. Pathfinder is super tall but not really wide. Revenant is not as tall, but his overall stature is pretty wide (look at how far apart his shoulders are from his head.) Octane has a similarly small size, comparable to Wraith, but what balances Octane is that his kit is designed for a risk vs. rewards type playstyle, but he also suffers from pretty linear movement patterns in order to take full effect of how fast he can go. Wraith on the otherhand is extremely mobile without the use of any abilities, and coupled with unpredictable movement makes her a hard target due to not being tall nor wide. If you made Wraith the same size as Lifeline, she'd probably be perfect.
So you'd think that her abilities would make her balanced, but they're really not. Her passive is always in effect and gives her and her team extremely valuable information. You might think that its weakness would be that you don't know where the shooter is, but just knowing there's a shooter gives you heaps of information, and simply scanning the area will 90% of the time reveal them. In comparison, Mirage's passive requires him to be downed, Octane's requires him to be hurt, Bangalore's requires her to be shot at, Revenant's is entirely useless, Gibralter's can easily be micromanaged to the enemies benefit, Caustic is useless without gas, Wattson's requires an item and is useless after using said item until her ulti is placed, half of Lifeline's passive requires a downed teammate and also is super bright and reveals her location and the other half requires her to be shot, Pathfinder's is useless during the first few rings and extremely hard to proc during the last few, Bloodhound's is pretty wonky and even when it works it's not entirely helpful, and Crypto's requires synergy with his drone, but if he's using his drone he's not moving, and if he's moving he can't mark enemies with his drone because it'll be stationary and an easy target to be destroyed completely.
What about her Q? It has a long cooldown, is easily followable, and isn't instant cast. But those are things you can work around, things you can minimize by getting more experience. Other characters have innate penalties given to their Q just by existing. Not all of them, but some of them. Bloodhound's Q reveals his location with every scan, Gibralter's dome is once again not the greatest ability, Caustic and Wattson require setup, Mirage Q is just completely useless, Lifeline's drone is not only stationary but good luck getting someone to drop the thing for you, Pathfinder's Q is like Wraith's but Pathfinder is much much taller than Wraith, Crypto is a stationary target in Q and good luck getting someone to cover for you if your team is moving, Rev's Q has a semi-slow projectile speed but the payoff is immense (if you hit the target or if you get them with splash, AKA you have to be good enough to hit someone which balances itself out), Octane's Q saps his health and he has to build up speed by running or sliding in order to become super fast, and Bangalore's smoke has the great tendency to be negated by digital threats or being see-through to everyone except you. It's not to say that you can't work around the above listed, it's just that in order to do so, you're reliant on positioning and luck rather than, "I'm low health and need to GTFO."
Lastly ulti? You might say again, long cooldown, and you can't shoot while using it. But it's a relocation ability, if you were allowed to shoot while deploying portal, they'd probably have to nerf something else. That flat 25% movement speed buff is pretty substantial too, making her pretty difficult to hit at any range given you have pretty good movement skills. Everyone else has innate penalties to their ultimates too, but not Wraith. Bangalore and Gibralter's ultis are slow but high damage and can easily be dodged, Caustic's takes a few moments to deploy and is also fairly easy to avoid given the right terrain and good reflexes, Octane's limits everyone to a linear movement pattern, Rev's forces you to heal once it ends and is easily followed, Wattson's confines you to a certain area in order to get full effect and is also limited by the fact it can be destroyed, Lifeline's is easily tracked across the map and is reliant on RNG in order to get actually good rewards, Pathfinder's ulti is pretty linear, loud, and easily shot whilst on, Bloodhound's is loud and the FoV change last I checked does mess with your sensitivity and makes him a pretty easy target to focus down (red eyes are scary), and Crypto's requires his drone which is easily shot down with shotguns, and makes him a stationary target.
