While there are many arguments here, I would like to sum them up together in a nice package that points out Ekko's flaws.
Firstly, Ekko as a champion at first glance has amazing mechanics, he seems to take lots of skill, timing and effort to pull off to his full potential. and this is correct. but his full potential is wiping out an entire team by himself after being shut down all game. when actually played, unlike lb or zed, or fizz, or many of the considered "overpowered" champions in the game already. it takes little to no actual experience and skill to play ekko. due to one simple fact. he has a total of three cc abilities. one of which has two cc's in it depending on how well it is used. he has a heal that at later games will completely reverse any punishment for assassinations, added to his low cooldowns that give little time of opportunity between his powerful poke and a full combo assassination. these factors all lead into the biggest problem. he has such a big kit. and it works even if you miss half of it. for example. if I miss my Q entirely, that is fine. I have a W waiting for them and I have an E that will close any range gap if I am even capable of wrapping my head around an age old League mechanic of dashing. and by the time I have hit them twice with an absolutely stupid amount of damage, without a passive proc. I now have another Q to launch. if they aren't dead, well, I can just ult and try again. now if that didn't make sense, that is fine. it is just a scenario. I am now going to run down his abilities:
Passive: not that bad honestly. standing alone, it is a three hit passive that does moderate damage and slows a bit, as well as giving him a speed boost. alone. this is a pretty balanced passive. even with his e, it is still balanced. what makes it unbalanced, is if you hit your q, it is an almost guaranteed that your E will pop it, slowing you and damaging you even more. and if you haven't built tank and it is at least mid game. you have died from just this combo. fine. he is an assassin that is okay.
Timewinder: This is an interesting skill shot, it seems to be low in range, and honestly the best way to avoid it is via hiding in minions as it slows based on when it hits things. so in lane it is not too hard to avoid if you are a mid laner. top it is harder but top you can build tank in the current Meta. regardless, this thing does an immense amount of damage with absolutely wonderful ap scaling, as well as slows you just perfectly for whatever else Ekko wants to do to you, personally, if I am hit by this, I find it very hard to move at all, and at the range it is at, as well as Ekko's painfully powerful mobility, he will get to me with little trouble. this ability in my opinion, is in truth the most unbalanced ability if standing alone in his kit, just due to it's sheer power and functionality, trying to catch someone? Q them, they aren't getting away. being chased? q them, they won't get you, and hey, maybe you can go in for a kill too. pushing a lane and it is stupid to jump into their team? just poke them with your q. this ability is very good and on such a low cooldown that it is hard not to broken.
Parallel Convergence: Personally, I believe this is his most balanced ability... as an active, here me out here. it is not an ekko fanboy here. while his ability does have amazing stun potential. as an assassin that requires getting in to their team, as well as guessing where the fight will be in three seconds. as an escape, this is pretty good for a turn around, but just pay attention, it is a very obvious skill and easy to see where it is going once you are used to fighting it. mechanically it is difficult to land this ability correctly. the only thing that needs fixing on it in my opinion is it's scaling, over all. Ekko's entire kit needs a look at it's total scaling, I know he is an assassin. but he is able to be up there with a half built akali in damage, with only a luden's echo or a lich bane. it is built in to him to do a lot of damage from a lot of sources, and to give him great scaling too is just too much. now the passive for this ability, is too much. with all of his kit, he does not need a low health damage boost. there is no reason for this. there is nothing lacking in his ability to kill low health players, nothing about him needs this passive to even exist.
Phase dive: This ability is a weird one to look at. it is a dash gap closer that allows him to have a single ranged attack that closes the gap. mechanically this is extremely simple to use. but to an enemy, this is extremely difficult to handle. when fighting against an ekko, it requires great skill and planning to engineer a fight that will not cause utter loss against him. and this is the biggest reason. Ekko's number one counter is being able to hold him still. but there has yet to be a champion (to my knowledge) that has succeeded in consistently being able to hold him still because of this ability. someone attempts to get a stun on ekko that isn't a target stun (which are few and far between to my memory). he can in most cases use this ability to not only avoid the attack as a good escape. but turn it around. if his q isn't doing damage to you, this ability will. because not only is it a great escape as well as a great gap closer, but it does great damage when paired with his kit. he can and will half health you with this ability paired with a lich bane. it is insanely strong. and versatile in use. a great gap closer and an amazing damage out put, it is very hard to avoid this bursting ability.
Chronobreak: This is the last piece of the puzzle that is Ekko's kit. this is the ability that takes a single game to get used to and use correctly. and I have to say, it does too much. this ability is not only a saving grace to any assassin, basically a free second chance to pop in to their team and kill someone. most assassins have their greatest weakness lie in after bursting down one enemy, they have to wait a while or they are entirely out of the fight, due to taking too much damage for it to be a good idea to go in again. Ekko's ult fixes this, it allows Ekko to completely remove all that damage done. he gets to go back and do it again. if this was all it did, this would probably be a well balanced ultimate, and a nice gift to an assassin. but instead, it has to do 130% ap scaling damage in an almost lane wide area of effect. that is correct, to any who didn't know the numbers, it does a 130% total ability power plus 200/500/700 damage to the area around him. to the average adc, ap carry, and non tank support/jungle. this is almost two thirds of their health if not more. this ability allows for Ekko to walk out of a fight unscathed, or. destroy an enemy team.
The biggest reason Ekko is overpowered. is that he can not be countered by a strategy, or a champion. you have to play against him hoping he will make a mistake, which is very hard to do. seeing as he has a kit with three slows, one stun, one gap closer, one heal, outrageous scaling and base damage, a dodge, a poke, easy one-shot capabilities. and a shield that scales 80% of his ability power, given to him just so long as he steps into his W, he doesn't have to land it. his Kit leads to unending possibilities of kiting, poking, assassination, Consistent damage, Amazing chase, Mobility, recover, and probably more I can not name. he has to much. and the worst part about it, is that it does not take skill to use. as mentioned before, he can up and miss half of his kit and still kill you. he is designed to be broken. he is a whole team in this meta, built into one, unkillable package. and if you do kill him. there are four other enemies to face, and I can guarantee that killing him will have dealt enough damage to your team to feel like you just went through a team fight. if he doesn't just Warp to your Carry as soon as they get in range to help kill him, one shot the carry, ult onto another carry, then hop and almost kill your third carry. I can not see how Riot, a company with large amounts of experience in Balancing champs, can publish such an obviously overpowered champion, and not even nerf him.
thanks for reading my rant, correct me where I am wrong, that is for the better, and here is to hoping he is nerfed for the fun of the LOL community.