You should probably study her patch update history before asking that question, it's really quite illuminating.
Eve's been back and forth between 'make newbies want to quit the game but obnoxiously difficult to use against veterans without playing teamfight janitor 24/7' to 'complete dog poo' to her current state, which still had to be nerfed to prevent her from 0-100ing people mid-game with DFG same as Ahri.
That's really just a very very brief summary of a development history you absolutely MUST be aware of before asking your question. Once you understand that, it becomes a little more obvious why people complain about assassin Eve. Assassin Eve doesn't initiate fights and doesn't contribute to damage, she pretty much just ults and cleans up. If she does anything besides that, she usually doesn't survive her teamfights.
Outside of her ult, she takes out one target and then feeds. Assassins are only well-liked in team comps if there is a very good likelihood they will survive blowing up their target. (See: Fizz, Kassadin, Ahri, etc.) If they are just doing one to one trades all day, then there will come a point the target buys armor and it no longer becomes a one to one trade and the assassin gets peeled and feeds.
Eve's Q, W and E are all DPS tools, they aren't really burst tools. That's why you're getting told to build Tank Eve. Is it the right answer? Not necessarily, it's situational.
