Sorta assassin..? Try top tier, one of the best assassins in the damn game. Near akali's level in terms of unavoidable assassination potential.
Don't have to sugar coat her strengths, when it comes to killin' nerds shes great at it.
As for her burst, it's more than Lb, zeds burst is situational and countered by QSS and HG, and talons burst is a lot different. Should be noted you're also comparing an AP to AD. Everything about em is far different, AD assassins get dunked on by hourglass/randiums/etc. Also, Diana is way more mobile than Talon, on top of that she has more utility compared to him. She has a nice shield, AoE pull in(can win you a team fight easily) and her damage is very unavoidable, unlike Zed(who needs ult) or Talon(who gets dumped on by pinks/can't lane well.)
She also has very strong wave clear/jung farm potential, and strong autos for an AP.
Why you trying to make people think shes bad? She isn't. She might not be top tier, but it doesn't mean every aspect of her kit is at fault. Is she a god tier assassin? You betcha. Is she a great farmer? No doubt. While she isn't as slippery as someone like Akali, her dmg/burst is similar. Shes borderline OP, man.
Let's talk about her real weaknesses, and not sugarcoat it saying shes overall weak.
First of all, yeah her burst is fucking nuts and she gap closes like a boss, but if she misses Q she can't R twice and a lot of her dmg is gone. Shes basically an akali who has to land the skill shot to be as effective.
She also can't really escape when she goes in(similar to most assassins, excluding lb/zed), but she does have hourglass(same as fizz/lb/akali/etc) and she has a nice shield, and a nice pull in ability.
Does she need buffs? Not really. They could change her into more of an AP bruiser, but the mechanic of Q>RR one shotting anyone would have to go, otherwise shes just as bad as Akali.