Depends how you look at the class.
Assassins should be bursting someone down and getting out, but her burst is the lowest in class, hence why she has the lowest TTK of all assassins and she still has a similar issue to prerework: can get in a fight, but not so easily out.
Yea, she has her W, but that's just buying time, unless it's near a wall she can hop with her E, she's not getting out, considering most probably her ult is down as it was used to kill someone.
As for outplay potential, that's subjective, since I don't consider LeBlanc's or Zed's "I 100-0 you before you can even toss out a spell" as outplay.
Here's some winrates in Diamond, random orders:
Zed: 49.5% wr, 12.1% pr, 26.4% br - avg kda: 8.4/6.3/5.3 - 2.17 - 13.7 KP
Fizz: 50.6% wr, 4.2% pr, 3.8% br - avg kda: 8/5.8/5.4 - 2.31 - 13.3 KP
Talon: 52.7% wr, 5.5% pr, 4.6% br - avg kda: 8.7/6.3/5.5 - 2.25 - 14.2 KP
Katarina: 50.1% wr, 5% pr, 2.8% br - avg kda: 9.2/6.6/5.5 - 2.22 - 14.7 KP
LeBlanc: 49.5% wr, 9% pr, 24.8% br - avg kda: 7.3/5.1/6.3 - 2.66 - 13.6 KP
Akali: 46.5% wr, 9.2% pr, 59% br - avg kda: 7/5.8/4.7 - 2.01 - 11.7 KP
She's not only losing more than any other mid lane Assassin, but also not even getting close to their KP.
Let's the rest of jungle assassins and Pyke, just for fun
Pyke: 49.5% wr, 17.2% pr, 32.8% br - avg kda: 6.6/6.1/8.6 - 2.49 - 15.2 KP
Kayn: 49.7% wr, 9% pr, 5.8% br - avg kda: 7.3/6.2/6.9 - 2.29 - 14.2 KP
Rengar: 50.8% wr, 7.7% pr, 5.6% br - avg kda: 8/6.2/5.7 - 2.20 - 13.7 KP
Bug: 52% wr, 16.6% pr, 13.1% br - avg kda: 8.4/5.6/5.9 - 2.55 - 14.3 KP
Evelynn: 51.6% wr, 5.2% pr, 14.4% br - avg kda: 8.4/5.5/7.0 - 2.8 - 15.4 KP
Get nerfed already aka Shaco: 53.7% wr, 5.2% pr, 7.1% br - avg kda: 7.8/5.1/6.7 - 2.84 - 14.5 KP
Nidalee: 50.9% wr, 8.7% pr, 5.1% br - avg kda: 7.2/5.5/7.4 - 2.65 - 14.6 KP
Oh hey, another invis that's not revealed by pinks and even Oracle doesn't let you click on him with the highest winrate of the Assassins and best kda ration. Let's cry about Akali who from stats is the worst performing assassin in Diamond. Barely wins, can't get as much KP as the rest, but Shaco? Nah, he's been S tier for like 9-10 patches now, but he's fine, he can oneshot you from invis then get away, but he's fine, Akali is the most offensive Assassin.
Is she frustrating to play against? Yes, she is.
Can someone who mastered her do better than other Assassins when mastered? Maybe, who knows?
Is she the best assassin out there? Far from it, heck, she's the worst at the job of them all. If I missed someone, feel free to say.