it's not so much that she's a trouble to play against. many heroes can be a trouble to play against.
it's that she does significantly more damage then any other ranged assassin while being significantly harder to kill.
consider her range compared to KT or Jaina. It's ludicrous. Compared to other heroes, she outranges everyone except for maybe hammer, raynor, or lunara... And then her abilities that outrange other heroes also do more damage and have a much shorter cooldowns. And then to top of all that she has a mobility tool with a 5 second cooldown.... this is the shortest cooldown on a mobility tool in the game... tied with Chen's flying kick which has to target a hero. To put it into perspective, they just removed blink from the other mages because it was deemed to powerful as a lvl 20 talent with a 60s cooldown... Li ming, the highest damage assassin in the game with the longest range and shortest cooldowns gets a better version of an OP lvl 20 talent in her base kit at lvl 1.
It's not a matter of learning how to play against her (we all have plenty of experiencing playing against her, she's in every game). Or even about picking counters to her. The game is about strengths and weaknesses, advantages and disadvantages, and she is simply better then the other heroes. she's unbalanced.