Basicly - Its broken.
With good runes and build it gets on 21 second cooldown.
His ult can literally one shot adc thru pd+death dance(Experienced it myself) while healing him like 3k hp.
This makes him poker, and that strongest one, he can one shot basicly anyone with q e combo, even if he fails he can just go back every 21 sec and repeat same thing.
Same ult makes him great engager if he goes zhonya(which is pretty much a must on ekko), just run into people, and when they start hitting you press zhonya, pretty much when allies come in you will be ready to ult someone and kill them soo fast they wont realize.
I probably shouldnt say it, but 21sec cd is gained with:
Ultimate hunter(max stacked): 25% ult dc
Cosmic insight + 40cd from build: 45% ult cd
In total 70% cd on ult.
Lvl 1 ult: we will ignore it
Lvl 2 ult: 90 seconds cd -70% = 27 seconds;
Lvl 3 ult: 70 seconds cd - 70% = 21 seconds;
There is alomst no way to kill this champ, especially without him doing shit lot of dmg. Only way would be insanely timed cc chained, if broke by 0.00001 milisecond he will ult back and again have 3k hp.
No. Counter. Play.
Edit: 30.6 Sec max cd